[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
Is this safe? Should be creating new `KeyValuePair` instead?

```c
      for (size_t i = 0, L = m1->length(); i < L; ++i) {
        *result << new (ctx.mem) KeyValuePair(m1->path(),
                                              m1->position(),
                                              (*m1)[i]->key(),
                                              (*m1)[i]->value());
      }
```