Mind-the-gaps

Gap.__invert__` makes poor assumptions about the bounds of a Gaps generic type. It's better not to make any assumptions and remove this operator. `PositiveInfinity` and `NegativeInfinity` are also removed with this change. Gaps can still be inverted "by hand", e.g. `my_gaps ^ Gaps([some_smallest_value < x, x < some_largest_value])`.

`Var` is now public. Useful if `x` is already in your namespace.

Create gaps from a string or from `x`, e.g.,
py
>>> from mind_the_gaps import Gaps, x
>>> 0 <= x
Endpoint(value=0, boundary='[')
>>> x < 1
Endpoint(value=1, boundary=')')
>>> print(Gaps([0 <= x, x < 1]))
{[0, 1)}
>>> print(Gaps.from_string("{[0, 1)}"))
{[0, 1)}

Mind the gaps!
Gaps are sets of mutually exclusive sorted intervals that can be union-ed, intersected, xor-ed, or subtract-ed.