Cleanup hashtable.h

Issue #26588:

* Pass the hash table rather than the key size to hash and compare functions
* _Py_HASHTABLE_READ_KEY() and _Py_HASHTABLE_ENTRY_READ_KEY() macros now expect
  the hash table as the first parameter, rather than the key size
* tracemalloc_get_traces_fill(): use _Py_HASHTABLE_ENTRY_READ_DATA() rather
  than pointer dereference
* Remove the _Py_HASHTABLE_ENTRY_WRITE_PKEY() macro
* Move "PKEY" and "PDATA" macros inside hashtable.c