Test for docstrings for public mappings

This introduces tests for the presence of docstrings in `tip.parser` and `tip.core` for all public mappings (`def` and `defn`.) The tests will fail if a public mapping fails to provide a docstring, but is equipped with an exception list in case future developers are hellbent on avoiding documentation for specific functions.

Seeing how the introduction of these tests made the test suite fail, I took it upon myself to document the failing mappings in the namespaces.

**BAD STYLE**: I also performed the completely unrelated task of renaming `tip.parser/grammar` to `tip.parser/parser`. This wasn't the place to do so, but it was done in profound and just outrage.

Accepting this pull request closes #11.