Update rand requirement from 0.7 to 0.8

[CAD97]

(no, I don't know why dependabot missed this one. Yes I'm using their PR format.)

Updates the requirements on rand to permit the latest version.

Changelog

Sourced from rand's changelog.

[0.8.0] - 2020-12-18

Platform support

• The minimum supported Rust version is now 1.36 (Bump MSRV to 1.36 rust-random/rand#1011)
• getrandom updated to v0.2 (Update getrandom to 0.2 rust-random/rand#1041)
• Remove wasm-bindgen and stdweb feature flags. For details of WASM support,
  see the getrandom documentation. (Add std_rng feature, remove deprecations rust-random/rand#948)
• ReadRng::next_u32 and next_u64 now use little-Endian conversion instead
  of native-Endian, affecting results on Big-Endian platforms (Fix next_u64_via_fill; rand_core changelog rust-random/rand#1061)
• The nightly feature no longer implies the simd_support feature (Disentangle nightly and simd_support feature rust-random/rand#1048)
• Fix simd_support feature to work on current nightlies (Fix "simd_support" feature rust-random/rand#1056)

Rngs

• ThreadRng is no longer Copy to enable safe usage within thread-local destructors (ThreadRng: fix use-after-free in TL-dtor; doc for sized iterators rust-random/rand#1035)
• gen_range(a, b) was replaced with gen_range(a..b). gen_range(a..=b) is
  also supported. Note that a and b can no longer be references or SIMD types. (gen_range doesn't take a Range rust-random/rand#744, gen_range(a, b) -> gen_range(a..b) and gen_range(a..=b) rust-random/rand#1003)
• Replace AsByteSliceMut with Fill and add support for [bool], [char], [f32], [f64] (Replace AsByteSliceMut with Fill rust-random/rand#940)
• Restrict rand::rngs::adapter to std (Restrict rand::rngs::adapter to std rust-random/rand#1027; see also Make libc dependency optional rust-random/rand#928)
• StdRng: add new std_rng feature flag (enabled by default, but might need
  to be used if disabling default crate features) (Add std_rng feature, remove deprecations rust-random/rand#948)
• StdRng: Switch from ChaCha20 to ChaCha12 for better performance (StdRng: Switch to ChaCha12 rust-random/rand#1028)
• SmallRng: Replace PCG algorithm with xoshiro{128,256}++ ( SmallRng: Replace PCG algorithm with xoshiro{128,256}++ rust-random/rand#1038)

Sequences

• Add IteratorRandom::choose_stable as an alternative to choose which does
  not depend on size hints (Add Iterator::choose_stable() rust-random/rand#1057)
• Improve accuracy and performance of IteratorRandom::choose (fix accuracy of IteratorRandom::choose() rust-random/rand#1059)
• Implement IntoIterator for IndexVec, replacing the into_iter method (Fix most clippy lints rust-random/rand#1007)
• Add value stability tests for seq module (Value stability tests for seq functionalty rust-random/rand#933)

Misc

• Support PartialEq and Eq for StdRng, SmallRng and StepRng (impl PartialEq, Eq for rngs rust-random/rand#979)
• Added a serde1 feature and added Serialize/Deserialize to UniformInt and WeightedIndex (Added serde1 feature to Serialize/Deserialize WeightedIndex rust-random/rand#974)
• Drop some unsafe code (Drop unsafe code from SeedableRng::seed_from_u64 rust-random/rand#962, Drop unsafe code from next_u64 rust-random/rand#963, Bump MSRV to 1.36 rust-random/rand#1011)
• Reduce packaged crate size (Reduce packaged crate size by 5kb using cargo diet -r rust-random/rand#983)
• Migrate to GitHub Actions from Travis+AppVeyor (Migrate to GitHub Actions rust-random/rand#1073)

Distributions

• Alphanumeric samples bytes instead of chars (Impl Distribution<u8> for Alphanumeric rust-random/rand#935)
• Uniform now supports char, enabling rng.gen_range('A'..='Z') (Implement SampleUniform for char rust-random/rand#1068)
• Add UniformSampler::sample_single_inclusive (gen_range(a, b) -> gen_range(a..b) and gen_range(a..=b) rust-random/rand#1003)

Weighted sampling

• Implement weighted sampling without replacement (Add SliceRandom::choose_multiple_weighted, implementing weighted sampling without replacement rust-random/rand#976, Re: weighted sampling without replacement  rust-random/rand#1013)
• rand::distributions::alias_method::WeightedIndex was moved to rand_distr::WeightedAliasIndex.
  The simpler alternative rand::distribution::WeightedIndex remains. (Move Alias-method WeightedIndex to rand_distr rust-random/rand#945)
• Improve treatment of rounding errors in WeightedIndex::update_weights (Rounding error fix on WeightedIndex::update_weights() rust-random/rand#956)
• WeightedIndex: return error on NaN instead of panic (Handle NaN in WeightedIndex with error instead of panic rust-random/rand#1005)

Documentation

• Document types supported by random (Update random() documentation rust-random/rand#994)
• Document notes on password generation (Remarks on password generation (2) rust-random/rand#995)
• Note that SmallRng may not be the best choice for performance and in some
  other cases ( SmallRng: Replace PCG algorithm with xoshiro{128,256}++ rust-random/rand#1038)
• Use doc(cfg) to annotate feature-gated items (Use doc(cfg) to annotate feature-gated items rust-random/rand#1019)
• Adjust README (README: what rand is / is not rust-random/rand#1065)

Commits visible in compare view.

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Closes #1113.

We shouldn't merge this until uuid upgrades rand, to avoid pulling in two versions of rand.

[cart]

I dig this, but we should hold off on merging until downstream dependencies like uuid move off of the old rand version, otherwise we'll fracture deps / compile multiple versions:

uuid-rs/uuid#504

[CAD97]

Yep. (If you dig a little bit, I kicked off the movement to get the uuid update published.) Uuid is the only dep pulling in rand:0.7 once bevy internal crates update, and the uuid update should be published "soon".

[cart]

Thanks for getting the ball rolling 😄

[CAD97]

uuid 0.8.2 with just the getrandom requirement has been published. Rebased; this should be good to merge.

cargo tree --duplicates after this change (and a cargo update) shows cfg-if as the only crate duplicated in-tree.