[MLton] MLton/MinGW installer improved

[Wesley W. Terpstra]

I've just finished a fairly major revision of the installer for
MLton/MinGW. There are several improvements:
  * Supports upgrades; previously you had to de-install before upgrading
  * Uses the same toolchain for both 32 and 64 bits (from the mingw-w64 project)
  * Keeps the complete toolchain, i.e. c++, fortran, and java
compilers (previously I only kept gcc)
  * Reduced installer size to 28MB (despite containing some 340MB!)
  * Obviously, it also includes the newest MLton from svn/HEAD

There are two versions of the installer, an msi and an exe. The
trade-off is that the msi is bigger (59MB vs 28MB), but the exe
requires 340MB of temporary space during the install (released upon
completion).