`<flat_map>`, `<flat_set>`: Unify `_Emplace_hint()`

`<flat_map>` and `<flat_set>` have very different implementations of `_Emplace_hint()`. They should be unified (not literally into the same function, but into very similar code patterns modulo the key/mapped difference).

There may be ways to further simplify them (I think there's too much special-casing of `_IsUnique` but I'm not certain).
