Test custom_derive integration

[killercup]

Many of the macro_rules! macros can be used with the custom_derive crate, e.g. BelongsTo! or Identifiable!. This is documented, by not tested.

We should (as a first step) add custom_derive as a dev dependency and expand the documentation code blocks to actually work as doc tests.

[shssoichiro]

I'd like to work on this one.

[killercup]

@shssoichiro Oh! You might want to hold off on this one right now. There are known bugs when using custom_derive! and we'll probably not spend much time fixing them and instead build something using macros 1.1.

[shssoichiro]

Got it! I'll check back later if this still ends up being necessary.

[sgrif]

Closing as it turns out custom-derive is completely broken and we're just going to remove it from the docs now that Macros 1.1 is a thing.