This commits includes new Mut methods which ensure that values get
correctly copied, if needed, following a btree Copy() operations.

This effictively allows for the BTree Copy method to create an isolated
snapshot that cascades to the interior values. Such as a BTree that has
nested BTrees as values.

Mut methods are only useful when all of the following are true:

- The interior data of the values require changes.
- The value is a pointer type.
- The BTree has been copied using `Copy()` or `IsoCopy()`.
- The value itself has a `Copy()` or `IsoCopy()` method.

Mut methods may modify the tree structure and should have the same
considerations as other mutable operations like Set, Delete, Clear, etc.

New methods include GetMut, ScanMut, AscendMut, DescendMut, ReverseMut,
IterMut, MinMut, MaxMut, etc.