Parse: Add quiet_parse functionality.
Parse test: Test quiet_parse.
AST test: Use quiet_parse instead of ParserDB.
Parse test: Use quiet_parse instead of ParserDB.
Type test: Use quiet_parse instead of ParserDB.
Parse test: Add trivial pattern test.
Drop ParserDB.
Parse test: Separate tests of Parser class and parse module.
Parse test: Test Parser.__init__.
Parse test: Groom code.
AST test: Fix flake8 warning.
Parse test: Fix flake8 warnings.
Parse test: Minor test refactor.
Merge pull request #70 from Renelvon/parse_api

Improve API and expand testing of parse module. Use the new API instead of ParserDB.
AST, Parse, Parse test: Separate constants from functions.

* Add ConstantDef node.
* Parse parametre-less function definitions as constants
* Require param_list to be non-empty (i.e. param_seq)
* Adapt parsing unit tests accordingly.