Rewritten query translator

[snej]

Total rewrite of QueryParser. The new version is called QueryTranslator. It is much, much cleaner and more extensible.

It operates in three passes:

1. Traverse the JSON/Fleece tree creating a tree of Node objects
2. Do a bit of postprocessing on the Node tree
3. Traverse the Nodes, writing SQL to an output stream.

There's (a bit) more documentation in docs/overview/QueryTranslator.md