Implements generators.

These are very basic at present, simply populating the enumeration state with successive evaluations. There are two major outstanding issues:

1. The generator is exhausted by the first enumeration; the traversal should instead take a generator provider that it calls at the beginning of enumeration.

2. There is no way for the generator to indicate that it has produced everything it intends to.
Generator traversals take providers.
Generators can tell their traversal that they have nothing further to generate.
Tuples can be copied.
Generator blocks receive their context as an argument.
RXGeneratorTraversal → RXGenerator.
Ignores the AppCode .idea directory.
Merge branch 'feature/generators' into develop