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.

How is offset designed to work?

Actually I'm not even sure what I'm doing is correct, because the docs on `ptr::offset` say that the given offset must be inbound, which negative ones technically aren't.