2nd combinator parser example

[Stephen Weeks]

> > This one is a text-file parser rather than a binary
> > parser (so you should even be able to test it with
> > your current mlton without my changes...).
> 
> Did you send the wrong file?  I tried to compile with an older MLton,
> but failed due to references to BinIO.

That's OK.  I was able to make it work by commenting out some stuff.
I have isolated the bug to one of our SSA optimization passes.  For
now, you can avoid the bug by compiling with "-drop-pass knownCase",
which turns off the buggy pass.  I'll let you know when we have a
fix