Use an "implicit" Context?

[waywardmonkeys]

It is tempting to move to using a thread_local!(static CONTEXT: Option<z3::Context> = None) and some helper functions to have a scoped Context ... this would let us remove a bunch (all?) of the Context parameters to the various functions in the z3 crate.

This would help make the API look more like the Python or JS APIs, while still letting people have multiple contexts if they need them.