Lambda-calculus

This release improves the string representations of `CollisionError`

This release improves `BetaNormalisingVisitor` and the Visitors for variable substitution:

- `BetaNormalisingVisitor` now yields alpha conversions
- add Visitor to perform unchecked substitution to improve performance when possible
- add ABC for visitors which can be applied lazily
- refactored Visitors for variable substitution to use shorter class names

This release adds method chaining for term construction

This release fixes the license display on PyPI (again)

This release fixes the license display on PyPI

First "real" release, to test uploading to pypi.