Add support for unicase::Ascii and uncased::Uncased

[thaliaarchi]

This PR adds full support for unicase::Ascii and the missing macro support for uncased::Uncased. Ascii is a newtype with no space overhead over &str, so is useful when Unicode-aware folding is not needed. The blocker for macro support with Uncased was lifted when Uncased::new was made const in 0.9.7. Tests pass.

[thaliaarchi]

This was unintentionally closed when I renamed master in my fork to unicase-and-uncased

[thaliaarchi]

The latest commit fixes an issue, where a macro_rules macro that calls phf_map! (etc.) would insert implicit parentheses around the call argument. This unwraps that group, so that phf_macros does not incorrectly reject those keys as invalid. An example of this pattern has been added as a test.

[thaliaarchi]

I've pulled master to resolve conflicts, because #291 does a subset of the changes I did in the second commit here.

Is there something blocking this PR?