[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)
```

PiperOrigin-RevId: 366498581

chex/_src/asserts.py

@@ -72,8 +72,9 @@ def _is_traceable(fn):
 
   tokens = (
       "_python_jit.",  # PyJIT  in Python ver. < 3.7
-      "_cpp_jit.",  # CppJIT in Python ver. < 3.7
+      "_cpp_jit.",  # CppJIT in Python ver. < 3.7 (deprecated)
       ".reraise_with_filtered_traceback",  # JIT    in Python ver. >= 3.7
+      "CompiledFunction",  # C++ JIT in jaxlib 0.1.66 or newer.
       "pmap.",  # pmap
       "vmap.",  # vmap
   )