FUNCT better types for the history log

This is backwards incompatible, but makes it possible to tell normal returns
from exceptions safely
FUNCT centralise history formatting in moka_history

Before this other modules need to know how to wrap exceptions and return
values.
FUNCT adapt moka_call_handler to the new history API
TEST adapt moka_call_handler_history_tests to the new history API
TEST adapt hold_state acceptance test to the new history API
TEST adapt moka_history_test to the new test history API

It is not covering exceptions yet
TEST test also exceptions in moka_history_tests
TEST adapt moka_fsm_test to the new history API
DOC small fixes in the readme