Correctly handle unpacked assignment for non-constant inside of function

[jpivarski]

Fixes issue #27 .

Old behavior incorrectly created multi-target assignment (Assign(targets=[x, y], value=1)) rather than unpacked assignment (Assign(targets=[Tuple(x, y)], value=Tuple(1, 2))) when the right-hand-side was not a constant inside a function.

• not a constant: because Python bytecode loads a constant expression as a whole, rather than playing with registers, as it must for a right-hand-side involving symbols
• inside a function: presumably different (correct) decompiling code handles the not-in-a-function case...

Includes a unit test called test_issue027.

[srossross]

Thanks, this looks great.