Implement parent selector & and selector functions

[robocoder]

• parent selector &, e.g.,

.foo.bar .baz.bang, .bip.qux {
  $selector: &;
}

where the value of $selector is now ((".foo.bar" ".baz.bang"), ".bip.qux").

• selector-nest()
• selector-append()
• selector-extend()
• selector-replace()
• selector-unify()
• is-superselector()
• simple-selectors()
• selector-parse()

[Cerdic]

For the record, what we have right now for the input

.foo.bar .baz.bang, .bip.qux {
  $selector: &;
  content: "#{$selector}";
}

is

.foo.bar .baz.bang, .bip.qux {
  content: ".foo.bar.baz.bang, .bip.qux"; }

[Cerdic]

parent selector is implemented in #693

[robocoder]

I've closed this for now as there is now basic support. However, there are some parent selector test cases in tests/scss_test.rb that are failing which will be addressed as devs (periodically) work towards increasing compatibility with the reference implementation.