Specialize into_heap() for Vec<T> when T is simple

[jorendorff]

Specialization isn't stable in Rust yet, but we can specialize the old-fashioned way: use the mighty if.

One of the IntoHeap traits should have a static method is_trivial() which, if true, means we can move a Vec<T> into the heap without copying the buffer. It's probably OK whenever Self::In=Self, so, for machine types, tuples of trivial types, structs and enums with all trivial fields...

Then the into_heap method for Vec<T> can just ask. The branch will be inlined away.

(It's probably UB to use that trick on a Vec<FooRef<'h>> to turn it into a Vec<Pointer<Foo>>, but maybe that shouldn't stop us.)