refactor(core): do not cache reverse-mapping of message types in RAM

Instead, walk the module for every message.

Also remove the custom wire type registration facility, which is not
used.

core/src/trezor/messages/__init__.py

@@ -1,43 +1,19 @@
+if False:
+    import protobuf
+    from typing import Type
+
+
+def get_type(wire_type: int) -> Type[protobuf.MessageType]:
+    """Get message class for handling given wire_type."""
     from trezor.messages import MessageType
 
-if __debug__:
-    from trezor import log
-
-if False:
-    from protobuf import MessageType as MessageType_  # noqa: F401
-
-    MessageClass = type[MessageType_]
-
-type_to_name: dict[int, str] = {}  # reverse table of wire_type mapping
-registered: dict[int, MessageClass] = {}  # dynamically registered types
-
-
-def register(msg_type: MessageClass) -> None:
-    """Register custom message type in runtime."""
-    if __debug__:
-        log.debug(__name__, "register %s", msg_type)
-    registered[msg_type.MESSAGE_WIRE_TYPE] = msg_type
-
-
-def get_type(wire_type: int) -> MessageClass:
-    """Get message class for handling given wire_type."""
-    if wire_type in registered:
-        # message class is explicitly registered
-        msg_type = registered[wire_type]
-    else:
-        # import message class from trezor.messages dynamically
-        name = type_to_name[wire_type]
-        module = __import__("trezor.messages.%s" % name, None, None, (name,), 0)
-        msg_type = getattr(module, name)
-    return msg_type
-
-
-# build reverse table of wire types
     for msg_name in dir(MessageType):
-    # Modules contain internal variables that may cause exception here.
-    # No Message begins with underscore so it's safe to skip those.
-    if msg_name[0] == "_":
-        continue
-    if msg_name == "utils":  # skip imported trezor.utils
-        continue
-    type_to_name[getattr(MessageType, msg_name)] = msg_name
+        # walk the list of symbols in MessageType
+        if getattr(MessageType, msg_name) == wire_type:
+            # import submodule/class of the same name
+            module = __import__(
+                "trezor.messages.%s" % msg_name, None, None, (msg_name,), 0
+            )
+            return getattr(module, msg_name)  # type: ignore
+
+    raise KeyError