Add a Map data type.

There are currently no operations for it.
Make list functions work for maps.

This adds Tree::Value#separator as a more generic way of getting the
separator of a SassScript value, since now maps notionally have a
separator as well.
Make empty lists function as empty maps as well.
Add SassScript functions for manipulating maps.
Support destructuring in @each.
Add reference docs for maps and @each.
Add a changelog entry about maps and @each.