Allow multiple register definitions and mid-circuit measurements

[rturrado]

Update libqasm version to 0.6.6.

libqasm parser can parse an AST with:

• multiple qubit/bit (register) definitions,
• references to qubit variables or qubit indices, and
• measure statements of the form b = measure q.

Parser:

• uses libqasm's default analyzer,
• filters out the Bit operands when converting the AST to IR.

RegisterManager:

• Remove bit register references.

Update tests to use measure instructions with bit operands.

Add test_multiple_qubit_bit_definitions_and_mid_circuit_measure_instructions integration test. Add Identity gate to the default gate set.