lnmsg: support both primitive and complex types (subtypes) in LNSerializer.

This renames lnmsg._{read,write}_field to lnmsg._{read,write}_primitive_field, renames
LNSerializer._{read,write}_complex_type to LNSerializer.{read,write}_field and allows
LNSerializer.{read,write}_field to handle both primitive and complex types.

Also makes these funcs public, as these encodings are used outside of lnmsg as well
(e.g. encoding blinded paths in BOLT12 invoice_request)

tests/test_onion_message.py

@@ -194,7 +194,7 @@ class TestOnionMessage(ElectrumTestCase):
 
         # TODO: serialization test to test_lnmsg.py
         with io.BytesIO() as blinded_path_fd:
-            OnionWireSerializer._write_complex_field(
+            OnionWireSerializer.write_field(
                 fd=blinded_path_fd,
                 field_type='blinded_path',
                 count=1,