Replace the previous caching/binary-search approach with
Newton-Raphson iteration and bisection fallback. This is the
same algorithm used by WebKit, Chromium, and Firefox.
The old code had a broken binary search comparator that could never
return 0 (the second condition was always true when the first was
false), leading to out-of-bounds vector accesses and crashes.
Fixes#3628.
Unbreaks Windows build. resolve_integer returns an i64 making creation
of a StepsEasingFunction with its result a narrowing conversion before
this change as a long is 32 bits on Windows.
This excludes `step-end` and `step-start` which are expected to be
converted to the equivalent function at parse time.
We are expected to serialize these as the explicit keywords - previously
we would parse as `EasingStyleValue` and serialize equivalent functions
as the keywords. This caused issues as we would incorrectly serialize
even explicit functions as the keyword.
This also allows us to move the magic easing functions to
`EasingFunction` rather than `EasingStyleValue` which is a bit tidier
