fixing test regressions

(cherry picked from commit 6b6f4cc9d76239f479cd3c38d8a80a9994d2da2c)
Merge branch 'new-world' of github.com:brownplt/pyret-lang into new-world
now that the concrete syntax of s-lams has changed, its tosource ought to change to match...

(cherry picked from commit e9e12c716d128a9479f92ad72d0d26dcd2a0a574)
fixing the edge case of reconstructing parse trees where a rule has a
right-nullable suffix consisting of multiple nonterminals and all of
them were empty, for constructUniqueParse and constructNextParse.
ConstructAllParses already handled this case...

(cherry picked from commit 02bedf8eb388d2570f54f5262fd31b01137deb52)