Replaced CVec with a new MallocBuffer struct.
Replaced CVec with a new MallocBuffer struct.

The old CVec with dtor stopped working when Send was made opt-in,
because the dtor must implement Send but the pointers weren't Send.
Since CVec will likely be removed from the std lib, I just replaced it
with a simple wrapper struct that implements Drop and AsSlice.

SSheldon/rust-objc,SSheldon/rust-objc,ngrewe/rust-objc,ngrewe/rust-objc