feat(managed): Add logic so retain can be used both on vec and smallvec

[tobias-wilfert]

While working on #5747 I would ideally have something like this irregardles of if attachments are a vec or a smallvec (items):

pub fn validate(attachments: &mut Managed<SerializedAttachments>) {
    attachments.retain(
        |attachments| &mut attachments.attachments,
        |attachment, _| utils::attachments::validate(attachment),
    );
}

[jjbayer]

nit: Make explicit that the wrapper and the underlying function have the same return type.

Suggested change

	fn retain_mut(&mut self, f: impl FnMut(&mut I) -> bool) {
	Vec::retain_mut(self, f);
	}
	}
	impl<I, const N: usize> RetainMut<I> for SmallVec<[I; N]> {
	fn retain_mut(&mut self, f: impl FnMut(&mut I) -> bool) {
	SmallVec::retain_mut(self, f);
	}
	fn retain_mut(&mut self, f: impl FnMut(&mut I) -> bool) {
	Vec::retain_mut(self, f)
	}
	}
	impl<I, const N: usize> RetainMut<I> for SmallVec<[I; N]> {
	fn retain_mut(&mut self, f: impl FnMut(&mut I) -> bool) {
	SmallVec::retain_mut(self, f)
	}