gh-148072: Cache pickle.dumps/loads per interpreter in XIData (GH-148125)

Store references to pickle.dumps and pickle.loads in _PyXI_state_t
so they are looked up only once per interpreter lifetime, avoiding
repeated PyImport_ImportModuleAttrString calls on every cross-interpreter
data transfer via pickle fallback.

Benchmarks show 1.7x-3.3x speedup for InterpreterPoolExecutor
when transferring mutable types (list, dict) through XIData.