Hm, this implementation seems allergic to passing types by value, which eliminates half of the allocations. It also makes the mistake of being mutable-first, and provides some fundamentally-inefficient operations.

The main mistake that this makes in common with most string implementations make is to only provide a single type, rather than a series of mostly-compatible types that can be used generically in common contexts, but which differ in ways that sometimes matter. Ownership, lifetime, representation, etc.