Remove unnecessary Clone bounds on Serialize impls

[jplatte]

The impls are not really useful for non-Clone types, but this allows deriving Serialize for types with fields of type imbl::Container<GenericParameter> without #[serde(bound)] overrides for the generates impls.

[jplatte]

Would be nice to do the same for Deserialize but that would require first removing the unnecessary Clone bounds for From<StdContainer<T>> for imbl::Container<T> / FromIterator<T> for imbl::Container<T>, which is probably not straight-forward.

[jneem]

Thanks!