doh! fixes issue 92 again.

tentative commit. this should solve issue 92, but isn't working, yet.

the idea is to taint any memoized follow set that hit a recursive loop,
directly or indirectly. the recursive calls (h_follow_rec) are allowed to use
these intermediates, but propagate the taint. the top-level routine (h_follow)
places the original taint on its result and may remove it at the end. it will
not return memoized results that are tainted (but compute them afresh).

this makes the computation more wasteful but avoids issue 92. intermediate
results could end up in the memoization table and be treated as final by
later calls to h_follow(). this problem could appear or not depending on the
order of nonterminals (i.e. pointers) in a hashtable.

NetBSD's character class test macros are defined such that they produce
a compiler warning when given a char argument. Adding a cast to the call
fixes this.

Refactored all of the desugaring code to no longer depend on memory being initialized to 0. Everything is about 12% faster now.

h_first(2, ...) won't give me length-2 strings