Obviously, we span versions both before and after that particular lint.

As of 1.45, it is stable (currently in beta). Removing need of nightly
feature and using a different named feature.

Backport to the 0.4 version, releasing as 0.4.8.

The assumption that the address of access's guarded reference stays the
same is not true.

Costs:
* The Map is now slower and adds an allocation.
* It can stop being Copy (but non-trivial guards weren't anyway) and
  it can stop being Sync/Send if the closure is not.
* The taken closure needs to be Clone.

Fixes #45.

Technically, it is a breaking change, but the plan is not to raise major
version, because:

* Even rust std gives exception to break compatibility for soundness
  hole fixes.
* It is not that likely people's code would break.
* Even if it breaks, they are much more likely to go to the fixed
  version then if the version got bumped and that's what they should be
  doing ASAP due to the potential UB.