[MLton] the syntax of MLBs

[Daniel C. Wang]

Not that I'm 100% happy with CM's approach but just for reference file names 
that are not quoted and use "/" are platform independent names.
Names that are quoted use the native file sytems syntax.

So you can write

bar/foo.sml or "C:\bar\foo.sml"

on a Windows machine.

I think we should let users have spaces in their file names and other native 
syntax, but we need not encourage there use by making it convenient.