Since Python 3.9 a call with a lone ** unpacking compiled to
BUILD_MAP 0 + DICT_MERGE 1 + CALL_FUNCTION_EX, copying the kwargs dict on
every call.  For vectorcall callees (all Python functions and most
builtins) that copy is wasted work: the dict is immediately unpacked onto
a flat argument vector.

The compiler now pushes the ** operand as-is.  CALL_FUNCTION_EX converts a
non-exact mapping to a dict itself (reusing the DICT_MERGE machinery, so
error messages are unchanged), and PyObject_Call copies the dict only on
the tp_call path, where the callee would otherwise receive the caller's
dict directly -- so a callee still can never mutate the caller's kwargs,
and the documented PyObject_Call/callable(*args, **kwargs) equivalence now
holds for the C API too (pythongh-86795).

_PyStack_UnpackDict takes a critical section while copying the dict's
items out, retrying if the dict was resized in between, which makes
unpacking a shared dict safe on free-threaded builds.

f(**d) with a small dict is ~1.4x faster; calls without ** unpacking are
unaffected.

Co-authored-by: Josh Rosenberg <1178095+MojoVampire@users.noreply.github.com>
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>