[MLton] mlnlffigen and filenames on Windows

[Matthew Fluet]

> While I haven't tested this, I think that given a C header with two things
> whose names only differ in case, mlnlffigen will goof and use the same
> filename for both.  I think that a reasonable way to fix this would be to
> generate only lowercase names in mlnlffigen and ensure that they are
> unique.  Below is a sketch of a patch to do this.  If this seems OK, I'll
> test it on Windows (MinGW) and commit it.

Seems fine to me.