[MLton] MinGW port mostly works

[Stephen Weeks]

The MinGW port mostly works.  Here are the regressions that fail.

* The following failures are OK.
  
  cmdline, command-line:
	command-line differences
  echo, signals, socket, suspend:
	unhandled exception: SysErr: Function not implemented [nosys]
	(fork not implemented, as with Cygwin)
  textio.2:
	different treatment of newlines (as with Cygwin)

* The following failures may or may not be OK

  filesys:
	unhandled exception: SysErr: Function not implemented [nosys]
	The call to Posix.FileSys.symlink.
  mutex, prodcons, signals2, thread2, timeout
	no setitimer
  unixpath:
	many problems
  world[1-6]:
	unhandled exception: SysErr: No such file or directory [noent]
	This is coming from the call to MLton.TextIO.mkstemps, which 
	calls Posix.FileSys.createf.  For some reason the call to 
	the open system call is failing.

I also made some basis library changes to fix the problems with the
date.sml and time.sml regressions.

Brent, please take a look at the failures and see if you can make any
progress, either by fixing them or explaining why they're OK.  I've
done everything to the port that I planned, so it's ready for your
next round of changes.  We also need to start thinking about what the
binary package should look like.