Speed up compilation by 22% by compiling serde_derive earlier

Enabling the "derive" feature of `serde` means `serde_derive` must
compile before `serde`. But `serde` does not depend on `serde_derive`.
Instead depend on `serde_derive` directly so 'serde' and 'serde_derive'
can compile in parallel. That speeds up compilation by 22%:

Before:

    $ hyperfine --prepare 'cargo clean' 'cargo build --all-features'
    Benchmark 1: cargo build --all-features
      Time (mean ± σ):      4.940 s ±  0.101 s    [User: 15.523 s, System: 3.558 s]
      Range (min … max):    4.773 s …  5.098 s    10 runs

After:

    $ hyperfine --prepare 'cargo clean' 'cargo build --all-features'
    Benchmark 1: cargo build --all-features
      Time (mean ± σ):      3.831 s ±  0.138 s    [User: 16.185 s, System: 3.602 s]
      Range (min … max):    3.606 s …  4.040 s    10 runs

Author: Enselic

Cargo.toml

@@ -27,12 +27,13 @@ rustdoc-args = ["--cfg=doc_cfg"]
 
 [dependencies]
 proptest = { version = "1.0.0", optional = true }
-serde = { version = "1", features = ["derive"], optional = true }
+serde = { version = "1", optional = true }
+serde_derive = { version = "1", optional = true }
 
 [dev-dependencies]
 bincode = "1"
 serde_bytes = "0.11.8"
 
 [features]
-serde1 = ["serde"]
+serde1 = ["serde", "serde_derive"]
 proptest1 = ["proptest"]

src/lib.rs

@@ -108,7 +108,7 @@ mod tests;
 /// Which method works best depends on what kind of situation you're in.
 // NB: Internal PathBuf must only contain utf8 data
 #[derive(Clone, Default)]
-#[cfg_attr(feature = "serde1", derive(serde::Serialize, serde::Deserialize))]
+#[cfg_attr(feature = "serde1", derive(serde_derive::Serialize, serde_derive::Deserialize))]
 #[cfg_attr(feature = "serde1", serde(transparent))]
 #[repr(transparent)]
 pub struct Utf8PathBuf(PathBuf);

src/serde_impls.rs

@@ -73,6 +73,7 @@ mod tests {
     use super::*;
     use crate::Utf8PathBuf;
     use serde_bytes::ByteBuf;
+    use serde_derive::{Deserialize, Serialize};
 
     #[test]
     fn valid_utf8() {