== More flexible get_or_intern ==
If we have an interner of Box<[u32]>, it would be nice
to have an API that allows us to pass a &[u32] to the
interner. Else, we'll have to create a box every time,
which is more expensive.
Limitations:
- In terms of Hashing / Equality comparison, we need to
take in count that Box<[u32]> and &[u32] may differ