simplify [p]xla.py by in-lining single-caller function
The functions xla._compile_jaxpr and pxla.compile_replicated only had one
caller each. The logic was confusing, and they had grown many arguments (and
needed more!).
By in-lining them we got a few other cleanups:
- remove _parameter_or_create_token in favor of a common xla._xla_callable_args
- fix a bug in the interaction between token inputs and arg tupling
- finally plumb the user function name through to XLA metadata (!!)
