Skeleton of mutable identical dictionary using built-in Map

[jpolitz]

This PR can be closed if it's not useful to have the discussion here, but I figured it's an easy way to talk about some of the issues/tests that we'd like to add.

[jpolitz]

@shriram this is something we talked about.

@blerner what do you think of this? I guess it needs a keys-now and likely remove-now to be truly useful.

I feel like there's enough reasons to have this, and it's straightforward given the existence of Map.