select on mutable hash tables

I vaguely recall this being discussed before, but couldn't find the issue: why doesn't `select*` work on mutable hash tables? It's true that the result might change at a later time, but the alternative (e.g. `select(keys h, blah)` or `selectKeys(new HashTable from h, blah)`) both seem aweful.

Start by locating the implementation and documentation for select* and mutable hash tables; the issue names no files or tests. Clarify whether selection should operate on mutable tables and what result-change semantics are expected, then define tests that establish when the work is done.

Written by the indexing model from the issue text.