core.common: move stats-related stuff to my.core.stats and add more thorough tests/docs

deprecate core.common.stat and core.common.Stats with backwards compatibility

my/core/util.py

@@ -1,6 +1,5 @@
 from pathlib import Path
 from itertools import chain
-from importlib import import_module
 import os
 import pkgutil
 import sys
@@ -15,17 +14,6 @@ def modules() -> Iterable[HPIModule]:
         yield m
 
 
-from .common import StatsFun
-def get_stats(module: str) -> Optional[StatsFun]:
-    # todo detect via ast?
-    try:
-        mod = import_module(module)
-    except Exception:
-        return None
-
-    return getattr(mod, 'stats', None)
-
-
 __NOT_HPI_MODULE__ = 'Import this to mark a python file as a helper, not an actual HPI module'
 from .discovery_pure import NOT_HPI_MODULE_VAR
 assert NOT_HPI_MODULE_VAR in globals()  # check name consistency