Allow omitted elements in destructuring tuple assignments

[cgewecke]

• RewritesVariableDeclarationTuple rule to parse de-structuring assignments correctly:

  var (,c) = (10, 20);
  var (d,,) = (10, 20, 30);

• Adds some examples to the tests
• Resolves #69

Gist of AST for this change

[federicobond]

LGTM