fbmessenger: add all.py, merge messages from different sources

followup for https://github.com/karlicoss/HPI/pull/179

my/fbmessenger/common.py

@@ -0,0 +1,46 @@
+from datetime import datetime
+from typing import Iterator, Optional, TYPE_CHECKING
+
+if TYPE_CHECKING:
+    try:
+        from typing import Protocol
+    except ImportError:
+        # requirement of mypy
+        from typing_extensions import Protocol  # type: ignore[misc]
+else:
+    Protocol = object
+
+
+class Thread(Protocol):
+    @property
+    def id(self) -> str: ...
+
+    # todo hmm it doesn't like it because one from .export is just str, not Optional...
+    # name: Optional[str]
+
+
+class Message(Protocol):
+    @property
+    def id(self) -> str: ...
+
+    @property
+    def dt(self) -> datetime: ...
+
+    @property
+    def text(self) -> Optional[str]: ...
+
+    @property
+    def thread(self) -> Thread: ...
+
+
+from itertools import chain
+from more_itertools import unique_everseen
+from my.core import Res
+def _merge_messages(*sources: Iterator[Res[Message]]) -> Iterator[Res[Message]]:
+    # todo might be nice to dump some stats for debugging, e.g. how many were overlapping?
+    def key(r: Res[Message]):
+        if isinstance(r, Exception):
+            return str(r)
+        else:
+            return r.id
+    yield from unique_everseen(chain(*sources), key=key)