Parsita

Addition of a few helper functions:
- `eof`: a parser that matches the end of source
- `success`: a parser that always succeeds with a given value
- `failure`: a parser that always fails with a given message
- `Result.or_die`: returns the value on success and raises and exception on failure

Initial release of parser combinator library for Python:
- Fundamental parsers: literal, alternative, sequential
- Common parsers: optional, discard left and right, regex
- Repeated parsers: rep, rep1, repsep, rep1sep
- Forward declarations inside context
- Standard sequence context `GeneralParsers`
- Whitespace ignoring context `TextParsers`
- Examples: expression, JSON
- Complete unit test suite
- Basic tutorial