[WIP] Add sass maps support

This PR implements the maps data type and the API as of 3.3.14. There were changes to the maps api in 3.4 none of which are implemented here.

#### TODO

- [x] map parsing
- [x] length
- [x] map-get
- [x] map-merge
- [x] map-remove (3.3 syntax)
- [x] map-keys
- [x] map-values
- [x] map-has-key
- [x] keywords

#### Known issues

- [ ] list and map variable keyword arguments
- [ ] ~~`@debug` support~~ - https://github.com/sass/libsass/issues/223

#### Possible issues

- [ ] have I sourcemaped correctly?