beartype.math API.

This commit by Harvard microscopist and general genius @tlambert03
defines a new public `beartype.math` subpackage for performing type
hint arithmetic, resolving issues #133 and #138 kindly also submitted
by @tlambert03. Specifically, this commit defines a:

* Public `beartype.math.TypeHint({type_hint})` class, enabling rich
  comparisons between pairs of arbitrary type hints. Altogether, this
  class implements a partial ordering over the countably infinite set
  of all type hints. Pedagogical excitement ensues.
* Public `beartype.math.is_subtype({type_hint_a}, {type_hint_b})`
  class, enabling @beartype users to decide whether any type hint
  is a **subtype** (i.e., narrower type hint) of any other type hint.

Thanks so much to @tlambert03 for his phenomenal work here.
(*Compelling compulsion of propulsive propellers!*)

beartype/_data/hint/pep/sign/datapepsigns.py

@@ -97,7 +97,7 @@
 HintSignAsyncIterable = _HintSign(name='Iterable')
 HintSignCoroutine = _HintSign(name='Coroutine')
 HintSignCollection = _HintSign(name='Collection')
-HintSignAsyncGenerator = _HintSign(name='Generator')
+HintSignAsyncGenerator = _HintSign(name='AsyncGenerator')
 HintSignAsyncContextManager = _HintSign(name='ContextManager')
 
 # Structural checks, a.k.a. protocols.

beartype/math/__init__.py

@@ -0,0 +1,3 @@
+from beartype.math._mathcls import TypeHint, is_subtype
+
+__all__ = ['TypeHint', 'is_subtype']