This will allow #19659 to use derive on display using:
#[parse(aliases = "-webkit-flex")]
Flex,
#[parse(aliases = "-webkit-inline-flex")]
InlineFlex,
And such.
The trait ToAnimatedZero now supports it, and it now applies to things with generics,
avoiding the trait bounds for field types of the variant on which it applies.
For now, only #[animation(error)] is supported on variants and it makes
both #[derive(Animate)] and #[derive(ComputeSquaredDistance)] ignore
this particular variant.
For now, only impls for types like in style::values::generics can be derived.
This also needed a few ToComputedValueAsSpecified impls that I would like to
replace by some #[to_computed_value(clone)] attribute, but I think it is ok
to keep it like this for now.
