This module is non-invasive to external code except the new method.
new
Imitate the btree_map/btree_map::Entry.
btree_map/btree_map::Entry
To solve the problem of unlimited memory usage, use this to replace the original in-memory BTreeMap<_, _>.
BTreeMap<_, _>
Iter over Mapx.
Returned by <Mapx>.get_mut(...)
<Mapx>.get_mut(...)