Move existing tests into a new __tests__ module under generator/lib.
Also define a new test for `util.new_context`, to prevent regressions as
changes are made.
This is an incremental change towards a strongly-typed util module,
aimed at reducing dependency on the DictObject class. The rough idea is
to annotate everything as Dict, add some tests to codify the existing
behavior, and then start defining dataclasses for the dischovery schema.
We also remove some unused logic & params.
Make a few changes in the toplevel src dir to help separate
templates from code. Specifically, we rename `src/mako` to
`src/generator`, and nest the mako templates inside a
`src/generator/templates` dir.
This isolates most Python code into the `src/generator/lib` dir.
