Add support for DEFGATE ... AS PAULI-SUM

[ecpeterson]

Description

Adds support for DEFGATE ... AS PAULI-SUM, which allows gate definition via an exponentiated Hamiltonian.

cf. quil-lang/quil#38 , quil-lang/quilc#498

Todo

• Check that the type relaxation in paulis.py is not too damaging.
• Check whether the gate is expected to be 'instantiated' in some way by pyQVM. (Compare against the behavior for DEFGATE ... AS PERMUTATION.)

Checklist

• The above description motivates these changes.
• There is a unit test that covers these changes.
• All new and existing tests pass locally and on Semaphore.
• Parameters have type hints with PEP 484 syntax.
• Functions and classes have useful sphinx-style docstrings.
• (New Feature) The docs have been updated accordingly.
• The changelog is updated,
  including author and PR number (@username, gh-xxx).

[appleby]

some comments for your consideration. no blockers, happy to merge.

[appleby]

if TYPE_CHECKING:
    import crazy_pills_amiright

[karalekas]

will need to review again but just very small stuff so far

[karalekas]

what does this do?

[appleby]

magic. see previous comment here and/or mypy docs here.

tl;dr it's an escape hatch provided by mypy to allow you to import a module when you need the types only for type checking purposes, but importing the module at runtime would result in an import cycle.