style: Ensure that derived types are right for optimized-away implementations.

We have this optimization where, for non-generic structs, we generate just a
clone / move as the ToComputedValue / ToResolvedValue implementation.

This moves the optimization a bit further down, and refines it so that we still
generate all the relevant where clauses that make it sound, that is, that all
the ToComputedValue implementations of the fields return the same type.

Otherwise this wouldn't be sound and the type would need to become generic.

We add an escape hatch (no_field_bound) for fields that need to be cloned but
which don't implement the trait. This is right now only for the RefPtr<> in the
shared font-family list, and a piece of code in PaintWorklet which looks kinda
fishy, and probably should be fixed (but we don't ship it in Firefox and there's
a pre-existing FIXME for servo, so I punted on it for now).

The other thing this patch does is adding a bunch of ToComputedValue /
ToResolvedValue implementations that are trivial and were missing.

Differential Revision: https://phabricator.services.mozilla.com/D67913

components/style/gecko_string_cache/namespace.rs

@@ -24,7 +24,7 @@ macro_rules! ns {
 }
 
 /// A Gecko namespace is just a wrapped atom.
-#[derive(Clone, Debug, Default, Eq, Hash, MallocSizeOf, PartialEq, ToShmem)]
+#[derive(Clone, Debug, Default, Eq, Hash, MallocSizeOf, PartialEq, ToComputedValue, ToResolvedValue, ToShmem)]
 #[repr(transparent)]
 pub struct Namespace(pub Atom);