shallow Clone for #[derive(Copy,Clone)]

shallow Clone for #[derive(Copy,Clone)]

Changes #[derive(Copy, Clone)] to use a faster impl of Clone when
both derives are present, and there are no generics in the type.

The faster impl is simply returning *self (which works because the
type is also Copy). See the comments in libsyntax_ext/deriving/clone.rs
for more details.

There are a few types which are Copy but not Clone, in violation
of the definition of Copy. These include large arrays and tuples. The
very existence of these types is arguably a bug, but in order for this
optimization not to change the applicability of #[derive(Copy, Clone)],
the faster Clone impl also injects calls to a new function,
core::clone::assert_receiver_is_clone, to verify that all members are
actually Clone.

This is not a breaking change, because pursuant to RFC 1521, any type
that implements Copy should not do any observable work in its Clone
impl.