Define a return instruction.

It is possible to return multiple values from a function, so ReturnData contains
a VariableArgs instance.

We don't want return instructions to appear as 'return (v1)', so tweak the
printing of VariableArgs so the parantheses are added externally.

meta/cretonne/formats.py

@@ -43,6 +43,8 @@ BranchTable = InstructionFormat(value, jump_table)
 
 Call = InstructionFormat(
         function, variable_args, multiple_results=True, boxed_storage=True)
+Return = InstructionFormat(variable_args, boxed_storage=True)
+
 
 # Finally extract the names of global variables in this module.
 InstructionFormat.extract_names(globals())