program.instructions is slow

[bramathon]

Pre-Report Checklist

• I am running the latest versions of pyQuil and the Forest SDK
• I checked to make sure that this bug has not already been reported

Issue Description

Each time the .instructions property of a program is called, the entire instruction list must be converted from rust. This makes accessing instructions by index very slow.

pyquil/pyquil/quil.py

Line 217 in 0b9489c

return list(self.declarations.values()) + _convert_to_py_instructions(self._program.body_instructions)