For the traits we derive which methods don't depend on associated types (i.e.
all of them but ToAnimatedValue and ToComputedValue), we now add trait bounds
for the actual field types directly, instead of bounding the type parameters.
For now, only #[animation(error)] is supported on variants and it makes
both #[derive(Animate)] and #[derive(ComputeSquaredDistance)] ignore
this particular variant.
