Avoid custom immutable map for headers. #231
Description
Currently we roll our own immutable map for headers.
The behavior is much like BuildMap from build_collection. I think we should replace our own implementation in favor of this, since it comes with a lot of extra niceties.
Alternatives:
- Consider using a proper immutable data-structure instead of the copy based approach of
build_collection. Fx. by using treap package. Argument against is that most maps are small and thus a copy is often faster in practice.