Make z3_ctx pub

[toolCHAINZ]

I'm working on writing Python bindings for some code that I have that uses z3.rs. I would like my bindings to be able to work seamlessly with python's z3 library (producing terms and assertions to be used in python's handle to a solver for instance). I'm writing an opinionated API around the assumption that Python is always "driving" and that we are ALWAYS using Python's global Z3 context (allocated once on first use, never dropped or moved).

Pyo3 has the requirement that any Rust type passing into Python cannot have a lifetime other than 'static, and all my structs have the z3.rs 'ctx lifetime.

I'm working around this by pulling in a pointer to Python's global Z3_Context and storing it in a thread_local RefCell<Z3_Context>. I have a second thread_local with a 'static reference to the contents of the RefCell.

If Context is #[repr(transparent)] then I believe I can safely transmute this reference into a &'static Context, which I can then use for all my FFI types.

I've already tried this and it seems to work fine, but to actually do this, I need the guarantee of transparent.

I don't think that making this change would break anything. I assume this is also a less dangerous change for z3.rs to make than allowing for constructing Context structs out of raw pointers in the high-level API.

[waywardmonkeys]

If the field was pub, would you be able to use it?

[toolCHAINZ]

Yes, that would work as well and be less gross! I'd just assumed there was a reason you didn't want to expose that.

[toolCHAINZ]

I've changed the PR to just make it pub, as that seems a lot cleaner.