Faster maps

This PR refactors the underlying data structure that supports maps. This fixes a couple open issues as well drastically improves the speed to map operations.

I'm fairly certain there's more speed to me gained in future iterations.

**note** this PR is still heavily a work in progress. Use at own risk.

This currently addresses: https://github.com/sass/libsass/issues/536, https://github.com/sass/libsass/issues/554,  https://github.com/sass/libsass/issues/556, https://github.com/sass/libsass/issues/557