Rollup merge of #161617 - maxdexh:can-i-has-map-allocator, r=nia-e

Add custom allocators to `(try_)map` on `Box`, `Rc`, `Arc`

Adds custom allocators to `(try_)map` on `Box`, `Rc`, `Arc`.

Not on `UniqueArc`/`UniqueRc` because too much allocator-related API is missing atm and I didn't want to end up writing unsound garbage. The `else` branch in those needs to drop a weakref, I'll get to that later.

cc rust-lang/rust#160534

r? nia-e