Add support for in-place map for `Vec`s of types with same size (take 3)

This is implemented using a new struct PartialVec which implements the proper
drop semantics in case the conversion is interrupted by an unwind.

For the old pull requests, see #15302, #16369.

@huonw: No, that `q.offset(-10)` is perfectly well defined if `p.offset(10)` was correct. It's within the bounds of the object it was based on.