[MLton] modifying flatten.fun to always flatten

[Stephen Weeks]

> I was wondering how you did your debugging, because make will
> fixpoint the compiler.

Actually, make will only fixpoint if you start with clean sources.  If
you have already built the compiler, then make will just do one round
of compilation, letting you test the feature you have added.
Although, if your feature is broken, you can't do this again, so you
should save the old build directory that worked to replace the broken
one.

> I am looking to simply build a version of the compiler so I can run
> simple test cases instead of having the compiler rebuild itself.

I usually begin small tests with a version of MLton built by SML/NJ,
which can be ready in under a minute.  Meanwhile, I start a self
compile running on another machine so that in a few minutes I will
have a fast version of MLton to run more tests or run all the
regressions.