Investigate having the generator impl traits manually instead of emitting `#[derive]`

[arnavion]

Investigate having the generator impl traits manually instead of emitting `#[derive]`

[Arnavion]

Replacing #[derive(Deserialize, Serialize)] with manual impls gives:

debug
master 5m08s 5m08s 5m22s
issue-4 3m08s 3m03s 3m02s

release
master 3m30s 3m30s 3m24s
issue-4 2m53s 3m01s 2m52s

The difference between debug-master and release-master is because proc macros like serde_derive are themselves compiled in debug mode for debug builds.

Replacing #[derive(Debug, Default)] with manual impls produces no discernible difference, probably because they're emitted by the compiler itself.

The only remaining dependency on serde_derive is for the impls for ::IntOrString. Replacing these with manual impls allows proc_macro2, quote, serde_derive and syn to be dropped entirely, bringing compile time down to 3m for debug and 2m30s for release.