[MLton] mlb support

[Henry Cejtin]

You  talk  about multiple paths leading to the same file because of symlinks.
There are other reasons that this can happen: you can  mount  the  same  file
system  multiple  times  (hopefully  read-only).   Also, much more typically,
there is the issue of paths which aren't full (i.e.,  do  not  begin  with  a
slash).   I  am  assuming  that  the  code will handle this last common case,
right?

Speaking of the error messages I mentioned before, I still  am  confused  why
the  first  message  wasn't  about the lexically earlier error.  It certainly
would be a bit better if we could have errors reported in this way  since  it
is  more what people expect.  Also once you get past the first error, you are
in a program which is not what was read (because that isn't a program) but is
in  what  the  error  correction  envisioned  as being what you meant, so the
errors are going to be stranger.