Allow reallocation to different alignment in `AllocRef` The allocator-wg [has decided](https://github.com/rust-lang/wg-allocators/issues/5#issuecomment-672591112) to support reallocating to a different alignment in `AllocRef`. For more details please see the linked issue. r? @Amanieu closes https://github.com/rust-lang/wg-allocators/issues/5