Additional MMaps implementations using efficient trees

[palmskog]

The paper Efficient Extensional Binary Trees by Appel and Leroy and its accompanying Coq sources describe the following efficient trees which could implement the MMaps interface:

• binary tries, using a canonical first-order representation; another minimalistic implementation here
• binary Patricia trees from section 12.3 of Functional algorithms, verified! by T. Nipkow et al. Not extensional.
• trie data structure with sparse, 256-degree nodes that branch on the next character of the string.