…e-threaded build

set_lookkey() is shared by callers that run without the set's lock and by
callers that already hold it, so it takes the critical section on a
mutable set itself.  set.discard() and set.remove() hold that lock, taken
by their Argument Clinic wrappers, and reached set_lookkey() through
set_discard_entry(), so every call re-entered the critical section and
went through _PyCriticalSection_BeginSlow().

Split set_lookkey() into a lock-held core and a locking wrapper, and use
the core from set_discard_entry(): all of its callers (set.discard,
set.remove, PySet_Discard and the difference-update helpers) hold the
lock, and the helpers already assert it.

Measured on the free-threaded build: s.discard(1); s.add(1) 34 -> 23 ns.

…ree-threaded build

KeepRef() takes the critical section on its target so that it is safe
from any caller, and for a type that keeps a reference to the stored
object it then calls PyCData_GetContainer(), which takes the critical
section on the root object.  The Simple.value setter already holds the
lock on self, taken by its Argument Clinic wrapper, and for a top-level
object the root is self as well, so `c_char_p().value = b"x"` entered the
same critical section three times and `c_int().value = 1` twice (the
"nothing to keep" early return sits inside the locked region).

Add PyCData_GetContainer_lock_held() for a caller that holds the root's
lock, use it from KeepRef_lock_held() when the target is its own root,
and call KeepRef_lock_held() from the setter.

Measured on the free-threaded build: c_int().value = 1 35 -> 22 ns,
c_char_p().value = b"x" 39 -> 22 ns.

…nts setter

The setter runs under a two-object critical section on the pointer and
the new target, then calls KeepRef() twice on the pointer and
GetKeepedObjects() on the target; both helpers lock their argument
again.  Call the lock-held variants instead, adding
GetKeepedObjects_lock_held() for the target.

Measured on the free-threaded build: p.contents = c_int(2) 86 -> 58 ns.

PyCData_set() already requires its caller to hold the critical section
on the destination object and asserts it, but finished by calling
KeepRef(), which takes that lock again.  Every store of a Python object
into a Structure or Union field, an Array item or a Pointer item went
through it.  Call KeepRef_lock_held() instead.

Measured on the free-threaded build: Structure int field assignment
38 -> 24 ns, c_char_p field assignment 69 -> 54 ns.

…tion re-entry fixes