add BindgenOptions::require_syn method

[pvdrz]

This PR adds the BindgenOptions::require_syn method that can be used to know if we need to parse the generated items with syn to do any code manipulation, like sorting semantically.

It also reorganizes the logic inside Bindings::generate so we can easily add different passes of code manipulation in the future.

[amanjeev]

I approve this but obviously we will wait for @emilio hehe

[kulp]

This seems reasonable to me.

[kulp]

Since we talked about syn before in #2251 and #2254, there seems to be little reason not to go this way.