gh-76785: Crossinterp utils additions (gh-111530)

This moves several general internal APIs out of _xxsubinterpretersmodule.c and into the new Python/crossinterp.c (and the corresponding internal headers).

Specifically:

* _Py_excinfo, etc.:  the initial implementation for non-object exception snapshots (in pycore_pyerrors.h and Python/errors.c)
* _PyXI_exception_info, etc.:  helpers for passing an exception beween interpreters (wraps _Py_excinfo)
* _PyXI_namespace, etc.:  helpers for copying a dict of attrs between interpreters
* _PyXI_Enter(), _PyXI_Exit():  functions that abstract out the transitions between one interpreter and a second that will do some work temporarily

Again, these were all abstracted out of _xxsubinterpretersmodule.c as generalizations.  I plan on proposing these as public API at some point.

Include/internal/pycore_runtime_init.h

@@ -95,6 +95,11 @@ extern PyTypeObject _PyExc_MemoryError;
               until _PyInterpreterState_Enable() is called. */ \
             .next_id = -1, \
         }, \
+        .xi = { \
+            .registry = { \
+                .global = 1, \
+            }, \
+        }, \
         /* A TSS key must be initialized with Py_tss_NEEDS_INIT \
            in accordance with the specification. */ \
         .autoTSSkey = Py_tss_NEEDS_INIT, \