hide several auxiliary synth functions
rm sys_open_fts2spec, merging its contents into sys_to_spec, same for env
use FTS.state_varname for the state variable in logic inside synth.synthesize

unless it is undefined - then use 'llc' and 'bloc'.
use dict keys as state variables in synthesize_many

to give the user full freedom to choose their variable names.
Improve associated docstring.
update docstrings and code style
update synth tests

now sys_to_spec and env_to_spec each require that statevar be passed as a positional arg.
add tulip ltl syntax section to user guide that defines EBNF

token precedence,
and substitutions of full operator names.