Covariant ReadOnlyView

[conradludgate]

ReadOnlyView is currently invariant over <K, V>. I've had at least one person ask me for help with this problem, and that's 1 more than I ever expected.

https://github.com/obi1kenobi/trustfall-rustdoc-adapter/compare/dashmap_issue

In my testing, removing the RwLock from the internals of ReadOnlyView fixes the above linked test case.

However, removing the RwLock requires a re-allocation. conradludgate@5e3afd8

Raising this issue to track whether this cost is acceptable, or if there's another work around, or if this is a "wontfix".