Now that EasingStyleValue is a lot nicer to use, there isn't much reason
to keep TimingFunction around.
(cherry picked from commit 7950992fc21e2428a7f32954bbe893a2b2d58cf7,
manually amended with the output of `git clang-format master`)
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.
(cherry picked from commit 667e313731f06fabf2a3f75893c3e8f15a4172be,
manually amended with the output of `git clang-format master`)
