[WIP] Add generic support for destructuring assignment in @each

This PR adds generic support for destructuring assignment in @each.

Fixes #492. Specs added sass/sass-spec#54, sass/sass-spec#98.

Todo

- [ ] https://github.com/sass/libsass/issues/577
This is just a hacked to get tests passing. This visitor needs to be updated correctly https://github.com/sass/libsass/issues/577#issuecomment-60706241