ChaCha20 (WIP)

This branch contains a WIP implementation of ChaCha20.

As this is the first code commit ever to the library, there are a large number of details that need to be fleshed out before this branch could be merged. This includes a number of general questions about the library's structure in addition to the code in question.

In addition, the coding rules (#2) have not been formalized. An initial version should be agreed upon before merging any code.