Create gaps from strings or from `x`.

Create gaps from a string or from x, e.g.,

>>> 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)}