Groovy: fix parsing of deconstructing assignment of existing variables

[greg-at-moderne]

What's changed?

Fixing Groovy parser way of handling of deconstructing assignment of existing variables. Previously only new variable declarations were handled.
e.g.

              def key = 'k'
              def elemAttrs = [:]
              (key, elemAttrs) = ['x', [:]]

What's your motivation?

Exceptions are thrown during parsing.