additional test coverage for HatsIdUtilities library

#58 refactored functions related working with hat ids into a library to be inherited into `Hats.sol`. This makes the contract more readable, and it also has the benefit of making those functions more readily available by other developers.

With that in mind, its important to ensure that the library works across a wider array of test cases than are narrowly required for `Hats.sol` alone.

Start by locating the HatsIdUtilities library and its inheritance or use in Hats.sol, then review issue #58 for the refactor context. Identify the existing tests and the hat-ID cases currently covered; done means the library has broader test coverage for the relevant cases without changing its intended behavior.

Written by the indexing model from the issue text.