Provide ST-like interface for building stuff

[garyb]

So for instance a Layer is created ST-style and then frozen, and can no longer be mutated. That way we get a pure interface safely, as you can only locally mutate while building, and then the effectfulness is restricted to actually setting layers on a chart, etc.

There may be a good reason why it's not possible to do this, but it's an idea anyway 😄

[beckyconning]

Or potentially build up some data structure and lenses (although ST certainly has benefits).