Lowers Map functions for RustQ.Meta.Lower.Stdlib, and types such calls.
On a HashMap or BTreeMap, get, fetch!, has_key?, put, delete, merge,
update/4, keys, values, to_list, and new keep Elixir's meaning: values read out
are copies, and an update takes the map only at its last use; see
RustQ.Meta.Lower.Collection. On a struct or a map with declared atom keys, get,
fetch!, has_key?, put, and replace! read and write its fields.