Reinstate support for user-defined gates in PyQVM.execute

[appleby]

In #874, PyQVM was changed to remove the restriction for
run-and-measure style programs. In the process, PyQVM.execute lost the
ability to execute programs with user defined gates, which apparently
broke pennylane-forest's PyQVM support.

This commit reinstates the block of code for extracting defined_gates
from the Program passed to PyQVM.execute, and with it the ability to
execute Programs with (non-parametric) defgates.

Fixes #1059

See also: PennyLaneAI/pennylane-rigetti#22

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.
• (Bugfix) The associated issue is referenced above using
  auto-close keywords.
• The changelog is updated,
  including author and PR number (@username, gh-xxx).

[karalekas]

Was that PR also what caused the PyQVM to get much slower? (re: PennyLaneAI/pennylane-rigetti#24) (cc: @ecpeterson)

[appleby]

PennyLaneAI/pennylane-rigetti#24

Other related pennylane-forest issues:

PennyLaneAI/pennylane-rigetti#21
PennyLaneAI/pennylane-rigetti#23