[WIP] Add sass maps support

#### TODO

- [x] basic map parsing
- [ ] advanced map parsing
- [x] length
- [x] map-get
- [ ] map-merge
- [ ] map-remove
- [x] map-keys
- [x] map-values
- [x] map-has-key
- [ ] keywords
The size is know at this point, is there any benefit to using it off the bat?

```x
Map* result = new (ctx.mem) Map(path, position, m1->length() + m2->length());
```