Eliminate import wrangling

[dtolnay]

Many of our modules have pretty nasty conditional imports.

https://github.com/dtolnay/syn/blob/6f03cfcdd9438fb50a7618882d8ec77f3f12cc2b/src/lit.rs#L9-L24

I believe Serde handles this better by having one shared module that contains all possible imports and is imported all at once by a glob import where needed.