Use underscore consts on rustc 1.37+

[dtolnay]

https://blog.rust-lang.org/2019/08/15/Rust-1.37.0.html#using-unnamed-const-items-for-macros

This would tend to produce more readable error messages, because the paths currently emitted by rustc in error messages are silly (rust-lang/rust#46991).

It will require adding a build.rs script to serde_derive similar to the one that exists in serde.