Consider additions from @ffwng's fork

[wrengr]

The disjoin function of https://github.com/ffwng/bytestring-trie/commit/735b605fdcc3665025f1f63c1389cd3127b76cae seems generally useful, and would also fit in with resolving issue #7 and PR #10. Unrelated to those issues/PRs, the cut function could also be helpful (though it needs a rename at the very least).