[JAX] Use a hand-written Python wrapper for CompiledFunction.

[copybara-service]

[JAX] Use a hand-written Python wrapper for CompiledFunction.

A hand-written Python wrapper gives us a number of benefits:

• it makes it cheaper to cast self to a CompiledFunction in the __call__ method, since we no longer need to use pybind11's dynamic casting logic.
• we can avoid building an empty kwarg dict even when no kwargs are passed, which is something pybind11 does behind the scenes.
• the change allows us to prepare for using faster calling conventions, e.g. METH_FASTCALL in Python 3.7 or newer, which would allow the interpreter to avoid forming a tuple for the positional arguments.

In passing, remove unused arguments from PopulateCacheEntry.

Benchmarks (N.B. incorporating the PyBuffer hand-written PyObject change as well):

name                 old cpu/op  new cpu/op  delta
jit_simple_dispatch  11.3µs ± 6%  10.0µs ± 8%  -11.17%  (p=0.000 n=20+20)

name                 old time/op             new time/op             delta
jit_simple_dispatch  11.3µs ± 6%             10.0µs ±10%  -11.25%        (p=0.000 n=20+19)