fields: Remove unnecessary parentheses

thanks, pylint!
schema: Remove redundant code.
Add "util" module and tests.

"util" collects commonly used stuff
abc: Improve docstring for SchemaABC
util: Add implementation of Python's contextlib.suppress.

For compatibility with Python 3.3 (which doesn't have suppress).
schema: Major cleanup (also update tests).

This change should make the code of SchemaMeta.__new__() cleaner. The
code also should test for more errors (mainly wrong __lima_args__), and
report those errors better (utilizing util.complain_about())
test_util: Add test.
schema: Cleanup Schema.__init__ a little.
Test insert position of __lima_args__["include"].
schema: Update docstrings.
Improve API doc & docstrings.
Merge branch 'feature/refactor' into develop