isinf xtests: I've decided isinf(x+oo) isn't a bug

we are not slaves to what SMT does, difference documented
safety check: copy_vars_to handles scalar bools
some commented bits to sym ctor
sym(bool) support, Issue #69
better logical support, Issue #58 and other things
bool assignments, mostly tests, no code changes

a fixme in there b/c of something that works, but I don't know why it does (filed Issue #71)
eq uses same code as lt, gt, etc
tests updates in lt