- Update `quote` and `syn` to parse new Rust syntax;
- Update `compiletest_rs` to solve
  Manishearth/compiletest-rs#86;
- Add support for arbitrary constant expressions as enum discriminants;
- Remove the need to `extern crate num` just for deriving.

This was made to bypass wrong type inference, such as `-10 as u64`.

Manishearth/compiletest-rs#93 fixed build on
nightly.

Will likely be able to parse even more new Rust syntax.

This transition required some API-incompatible changes.

I couldn't find a way to use "full-syntax" only for `with_custom_values`
:(.