Improve unmarshall performance

This change reduces the overhead of unmarshalling with the
goal of having bleak scanners not overwhelming the system.

See related issue:
hbldh/bleak#236 (comment)

```
(speed_up_unmarsh) % python3 bench/unmarshall.py
Unmarshalling 1000000 bluetooth rssi messages took 14.397348250000505 seconds
(master) % python3 bench/unmarshall.py
Unmarshalling 1000000 bluetooth rssi messages took 47.7756206660124 seconds
```

bench/unmarshall.py

@@ -0,0 +1,22 @@
+import io
+import timeit
+
+from dbus_next._private.unmarshaller import Unmarshaller
+
+bluez_rssi_message = (
+    "6c04010134000000e25389019500000001016f00250000002f6f72672f626c75657a2f686369302f6465"
+    "765f30385f33415f46325f31455f32425f3631000000020173001f0000006f72672e667265656465736b"
+    "746f702e444275732e50726f7065727469657300030173001100000050726f706572746965734368616e"
+    "67656400000000000000080167000873617b73767d617300000007017300040000003a312e3400000000"
+    "110000006f72672e626c75657a2e446576696365310000000e0000000000000004000000525353490001"
+    "6e00a7ff000000000000"
+)
+
+
+def unmarhsall_bluez_rssi_message():
+    Unmarshaller(io.BytesIO(bytes.fromhex(bluez_rssi_message))).unmarshall()
+
+
+count = 1000000
+time = timeit.Timer(unmarhsall_bluez_rssi_message).timeit(count)
+print(f"Unmarshalling {count} bluetooth rssi messages took {time} seconds")

dbus_next/_private/constants.py

@@ -16,3 +16,8 @@ class HeaderField(Enum):
     SENDER = 7
     SIGNATURE = 8
     UNIX_FDS = 9
+
+
+HEADER_NAME_MAP = {
+    field.value: field.name for field in HeaderField
+}