set default args in synthesize, not deeper
allow user to select mutex/xor/none, int/bools for actions

by 2 new synthesize args: bool_actions, actions_must
update synth.is_realizable signature

extract default names of action variables from synth.synthesize to synth._default_action_vars
update tests to new sys_to_spec, env_to_spec signatures
clean unrealizable synth test of redundant comments