Update dependencies and clean up the code

[koute]

This PR cleans up the schnorrkel code a little to prepare for a new release.

• Switched back to curve25519-dalek from curve25519-dalek-ng (it turns out using curve25519-dalek-ng was a bad decision as that fork is effectively dead now)
• Updated to Rust 2021
• Removed old cruft and dead code
• Fixed all clippy lints
• Updated all of the dependencies
• Cleaned up the feature flags a little
• Marked musig as deprecated (as requested by @burdges)
• Bumped version to 0.11.0

This should not yet be released on crates.io. Before we cut a new release I want to do two more things:

1. Wait until my PR adding runtime SIMD autodetection is merged in curve25519-dalek and a release is cut, and then bump schnorrkel to use that. This should provide massive speedups in the range of ~30%-~50%.
2. Do more testing. I'm going to run a full substrate burn-in to make sure nothing's accidentally broken. I might also do some fuzz testing to make sure the behavior's equivalent to the 0.9.1 version currently used by substrate.

[koute]

I've also added a basic Github Workflows config to have some elementary semblance of a CI.

[burdges]

LGTM