Existence check for variables.

It used to be very hard to detect this condition and caused a
side-effect:

    $maybe-exists: some-value-it-should-not-have !default;
    @if $maybe-exists == some-value-it-should-not-have {
      // do something if not set.
    }
    // you can no longer default the variable :(
Existence check for global variables.
Sass function to test if a function exists.
Sass function to test if a mixin exists.
Address code review comments
Make variable-exists() work correctly for local variables.
Pass a read only environment of the caller to ruby functions.
Address final code review comments.
Address rubocop errors.
Merge branch 'existence_checks'