Add support for tuples

[chuckwondo]

The SandboxTransformer does not support "tuple" assignment (object destructuring with multiple assignment), as per the following:

groovy-sandbox/src/main/java/org/kohsuke/groovy/sandbox/SandboxTransformer.java

Line 729 in 01744cc

throw new UnsupportedOperationException("not supporting tuples yet"); // cf. "Unexpected LHS of an assignment" above

Is there any plan to implement this?