Rule-engine

* Added the `is_nan` attribute for `FLOAT` values

* Added attributes for coercion of types to themselves, e.g. `to_str` for `STRING` values

* Added the `$abs` builtin function
* Added support to `from_type` Python’s type hints

- **Breaking:** Changed `STRING.to_ary` to return an array of characters instead of splitting the string
- Use the new builtin `$split` function to split a string on whitespace into an array of words
- **Breaking:** Changed `Context` to use keyword-only arguments
- **Breaking:** Dropped support for Python versions 3.4 and 3.5
- **Breaking:** Invalid floating point literals now raise `FloatSyntaxError` instead of `RuleSyntaxError`
- **Breaking:** Moved `rule_engine.engine.Builtins` to `rule_engine.builtins.Builtins`
- Added the new `FUNCTION` data type