cut dependence of LetScoper on prim._let

the dependency is moved to IdentifierParser instead.  in a way this
only postpones the problem since eventually we want to cut
IdentifierParser's engine dependencies too, but hey, it's progress.
fix bug in ruler.scala lines-of-code counting script

it got confused if there were digits in a package name, e.g. 'parse0'
rename TokenType constants to follow Scala convention