This allows us to not generate trait bounds for field types anymore,
by excluding bounds for type parameters used only in fields annotated
with `#[animation(error)]`.
The syntax is `#[animation(no_bound(Foo, Bar))]` where `Foo` is a type
parameter for the type on which we derive things. Ideally this should
be an attribute on the type parameter but this isn't stable yet.
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.
