Avoid custom immutable map for headers.

[nielsenko]

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.