v0.0.23: Prelude: Map improvements (#523)

* Prelude: Map improvements

Use `uninitialized` intrinsic to have "empty" `MapEntry` objects in the
`Map`'s `_entries` field, rather than the previous approach of having an
array of `MapEntry?` values. This approach adds a bit more logical
overhead, but it significantly reduces the amount of allocations when
working with `MapEntry` values, and also eliminates the need for
`flattenOption`.

* Updating process_callstack test, because the contents of prelude changed