Apple Clang doesn't like this, rather than waiting for their version of
random-clang-commit-to-call-a-release to catch up with llvm trunk, just
work around the issue.
Fixes#186.
The values aren't that complex, so it doesn't make much sense to have a
dedicated generator for them. Parsing them manually also allows us to
have much more control over the produced values, so as a result of this
change, EasingStyleValue becomes much more ergonomic.
