modularize wallet classes

- all code has been relocated from `wallet.py` to individual modules under
  `wallet`, with each wallet type having its own module

- the fully rewritten initialization code can be found in `__init__.py` and
  `base.py`

setup.cfg

@@ -54,6 +54,7 @@ packages =
 	mmgen.share
 	mmgen.tool
 	mmgen.tx
+	mmgen.wallet
 
 scripts =
 	cmds/mmgen-addrgen