Remove implicit conjunction

I thought that since "tcp port 80" should parse as "tcp and port 80" that in fact the implicit "and" that the pflang spec mentions as obsolete was in fact current.  This faulty conclusion was based on my bad understanding of how "tcp port 80" parses -- that in fact as issue #15 indicates, "tcp port 80" is one operator.

So, we should remove the implicit conjunctions from the parser.
OK, I though it was mostly fixed due to #15. The point is that now when running "make check" the else branch is never executed, so that means for all the tested expressions 'prec' is never nil, but even in that the case the patch should have removed the else branch and the check.

So would it still possible that there will be an expression for which 'prec' is nil? I will try to research more on this.