C API: String Context

[roberth]

Is your feature request related to a problem? Please describe.

The string context is not accessible yet.

Describe the solution you'd like

One or both of

• C functions to read the string context from a string value.
• C function to realise a string context, and perhaps a function to merge contexts

Describe alternatives you've considered

Additional context

8699

Priorities

Add :+1: to issues you find important.

[roberth]

Made a start with C functions to read the string context from a string value, but doing realisation first is more bang for the buck.

[Ericson2314]

@pennae had brought up reducing heap data structure <-> std::set<std::string> round trips in the evaluator. Perhaps that could inform this.