Further consolidate rules under "single node"

"single node" is really quickly becoming "ruby value", but since EVERYTHING
is a ruby value ... I'm hesitant to jump there.

Would be nice to take a look at the MRI parser and see how close some of this is.
Improve handling of unless ... end expressions

There was some potential confusing as to whether the arguments to a call
expression should be parsed as a group inside the body of the unless.

Obviously that's completely wrong. The correct thing to do is to state that
there MUST be some sort of line separate at the end of the unless conditional
and before the list of statements that make up its body.