Implement `reserve_len` and `reserve_len_exact` for VecMap

Implement `reserve_len` and `reserve_len_exact` for `VecMap` in accordance with rust-lang/rfcs#509.
So then would the correct wording for VecMap be to change the first sentence to
"Reserves capacity for the given `VecMap` to contain `len` distinct elements..." and then keep the rest of what I had, or should the second sentence be changed to mirror BitvSet too?  Can't quite tell what would make sense