Add 'MPAD' instruction

[rafaelha]

See here.

The 'MPAD' Instruction
Pads the measurement record with the listed measurement results.

This can be useful for ensuring measurements are aligned to word boundaries, or that the number of measurement bits produced per circuit layer is always the same even if the number of measured qubits varies.

Parens Arguments:

If no parens argument is given, the padding bits are recorded perfectly.
If one parens argument is given, the padding bits are recorded noisily.
The argument is the probability of recording the wrong result.
Targets:

Each target is a measurement result to add.
Targets should be the value 0 or the value 1.
Examples:

# Append a False result to the measurement record.
MPAD 0

# Append a True result to the measurement record.
MPAD 1

# Append a series of results to the measurement record.
MPAD 0 0 1 0 1