Allow DSLX-specified unit tests to run through the JIT as an execution engine about xls

[cdleary]

Something we noted in chatting is that there are not yet Python bindings for running the LLVM XLS JIT, so a subtask here is to create Python bindings to the Run function there similar to the ones for the IR interpreter.

[cdleary]

Closing this out, though going to create a derived issue for the (smaller task) remaining bits:

1. We don't translate the test function body itself (the one generally containing assert_* builtin calls), only function calls (via Invocation nodes) that are reached from the test function entrypoint.
2. We don't reuse JIT objects via our Python bindings; e.g. would be useful to amortize overhead in QuickCheck, we have to re-JIT every time at the moment I believe.