bpo-40609: Rewrite how _tracemalloc handles domains (GH-20059)

Rewrite how the _tracemalloc module stores traces of other domains.
Rather than storing the domain inside the key, it now uses a new hash
table with the domain as the key, and the data is a per-domain traces
hash table.

* Add tracemalloc_domain hash table.
* Remove _Py_tracemalloc_config.use_domain.
* Remove pointer_t and related functions.

Author: vstinner

Include/internal/pycore_pymem.h

@@ -88,17 +88,12 @@ struct _PyTraceMalloc_Config {
     /* limit of the number of frames in a traceback, 1 by default.
        Variable protected by the GIL. */
     int max_nframe;
-
-    /* use domain in trace key?
-       Variable protected by the GIL. */
-    int use_domain;
 };
 
 #define _PyTraceMalloc_Config_INIT \
     {.initialized = TRACEMALLOC_NOT_INITIALIZED, \
      .tracing = 0, \
-     .max_nframe = 1, \
-     .use_domain = 0}
+     .max_nframe = 1}
 
 PyAPI_DATA(struct _PyTraceMalloc_Config) _Py_tracemalloc_config;