Replace syntax::opt_vec with syntax::owned_slice

Replace syntax::opt_vec with syntax::owned_slice

The `owned_slice::OwnedSlice` is  `(*T, uint)` (i.e. a direct equivalent to DSTs `~[T]`).

This shaves two words off the old OptVec type; and also makes substituting in other implementations easy, by removing all the mutation methods. (And also everything that's very rarely/never used.)