Create indexmap/set in static contexts

[mwillsey]

Would it be possible to add a new-like function that is const? It would be useful to create static maps without the aid of something like once_cell.

From scanning the code, it seems like with_hasher is maybe the right function to make const, since you don't have to make a hasher (it's given) and "no allocation" is already in the spec for that function.

[cuviper]

Since we now have const Vec::new and RawTable::new as well, yes I think this should be possible.

[mwillsey]

Happy to make a PR. Do you think with_hasher is the right target? Any others?

[cuviper]

A PR is welcome! I think with_hasher is the only constructor we can support. For precedent, the standard HashMap doesn't have any constant functions, but hashbrown does have constant with_hasher in its own map and set.

For other methods, Vec and/or RawTable don't give us anything else to work with, not even len.