[MLton] mlb support

[Matthew Fluet]

> > Certainly copying the basis on installation will change the inode number
> > of files.  That would suggest that we have a post-install operation to
> > read in .mlbs and output a world with them parsed/elaborated.
>
> I'm getting confused.  I thought we were just talking about a hack to
> speedup an SML/NJ-compiled MLton, which has no bearing on the binary
> MLton packages that we distribute and hence no need for post-install
> stuff.  Why do we need to cache bases across runs of a MLton-compiled
> MLton?

You are right, we don't need to cache bases across runs of a
MLton-compiled MLton.  But, I'd still prefer to do the "right thing" up
front.