Support const int for register sizes

[TheGupta2012]

Summary of changes

Register Sizes

• Compile time constants are valid expressions for quantum and classical types but const int[32] N = 2; qubit[N] q; was not being parsed correctly
• This is because sizes were assumed to be integer literals when they could be arbitrary expressions involving compile time constants
• Changes made to _visit_qubit_register and _visit_classical_declaration ensure that size value is correctly evaluated for classical and quantum registers

Bug in _visit_function_call

• return_value parameter was undeclared and was not correctly returning value for functions which did not have a return_statement field