Revert `Vec::spare_capacity_mut` impl to prevent pointers invalidation
The implementation was changed in #79015.
Later it was [pointed out](https://github.com/rust-lang/rust/issues/81944#issuecomment-782849785) that the implementation invalidates pointers to the buffer (initialized elements) by creating a unique reference to the buffer. This PR reverts the implementation.
r? ```@RalfJung```