run_symmetrized_readout does not respect memory declarations

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

QuantumComputer.run_symmetrized_readout produces symmetrized programs which may contain duplicate memory references. Quil does not allow for duplicate memory references.

Example

>>> import pyquil
>>> from pyquil import Program, get_qc
>>> from pyquil.gates import H, MEASURE
>>> from pyquil.quilbase import Declare, MemoryReference
>>> qc = get_qc('3q-qvm')
>>> qc.run_symmetrized_readout(Program(Declare('ro', 'BIT'), H(0), MEASURE(0, MemoryReference('ro', 0))), 1000)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/erik/src/pyquil/pyquil/api/_error_reporting.py", line 238, in wrapper
    val = func(*args, **kwargs)
  File "/Users/erik/src/pyquil/pyquil/api/_quantum_computer.py", line 210, in run_symmetrized_readout
    results = _measure_bitstrings(self, sym_programs, meas_qubits, num_shots_per_prog)
  File "/Users/erik/src/pyquil/pyquil/api/_quantum_computer.py", line 919, in _measure_bitstrings
    prog = qc.compiler.quil_to_native_quil(prog)
  File "/Users/erik/src/pyquil/pyquil/api/_error_reporting.py", line 238, in wrapper
    val = func(*args, **kwargs)
  File "/Users/erik/src/pyquil/pyquil/api/_compiler.py", line 340, in quil_to_native_quil
    response = self.client.call('quil_to_native_quil', request, protoquil=protoquil).asdict()  # type: Dict
  File "/Users/erik/miniconda3/envs/pyquil/lib/python3.7/site-packages/rpcq/_client.py", line 180, in call
    raise utils.RPCError(reply.error)
rpcq._utils.RPCError: At line 5: The name "ro" has been DECLAREd more than once.

Start in pyquil/api/_quantum_computer.py at run_symmetrized_readout and follow its call to _measure_bitstrings, both named in the traceback. Reproduce the example with a declared ro memory region and verify that the generated symmetrized programs no longer contain duplicate memory declarations or trigger the compiler error.