Decouple serde from its derive crate

[CryZe]

By not activating the derive feature on serde, the compilation speed can be improved by a lot. This is because serde can then compile in parallel to serde_derive, allowing it to finish compilation possibly even before serde_derive, unblocking all the crates waiting for serde to start compiling much sooner.

As it turns out the main deciding factor for how long the compile time of a project is, is primarly determined by the depth of dependencies rather than the width. In other words, a crate's compile times aren't affected by how many crates it depends on, but rather by the longest chain of dependencies that it needs to wait on. In many cases serde is part of that long chain, as it is part of a long chain if the derive feature is active:

proc-macro2 compile build script > proc-macro2 run build script > proc-macro2 > quote > syn > serde_derive > serde > serde_json (or any crate that depends on serde)

By decoupling it from serde_derive, the chain is shortened and compile times get much better.

Check this issue for a deeper elaboration:
serde-rs/serde#2584

For samply I'm seeing a reduction from 5.43s to 3.70s when compiling fxprof-processed-profile in release mode.

The impact is even larger for crates that depend on fxprof-processed-profile such as wasmtime.

[mstange]

That's a great result! Thank you!