Eliminate syntect library's dependency on serde's "derive" feature

[dtolnay]

Here are charts from cargo build --timings.

Notice how Before, syn -> serde_derive -> serde -> {bincode,serde_json,plist} -> syntect is a bottleneck in the build, whereas After, nothing related to serde is on the critical path. Instead, libc -> cc -> onig_sys -> onig -> syntect is the new bottleneck.

The build is 15% faster end-to-end (7.6 seconds vs 8.9 seconds). More importantly, notice how if whatever downstream crate that depends on syntect has other dependencies that depend on serde, those can begin building after only 2.6 seconds, instead of after 6.3 seconds, which can speed up builds by up to 3.7 seconds, including making IDEs more responsive by that amount.

Before	After

[Enselic]

Nice, thanks!