[MLton] C-types

[Matthew Fluet]

> I do plan on fixing the signedness issue for the use of C-types in the Basis 
> Library, although it probably won't be this evening.
>
> I believe that the c-types.sml files are being generated properly -- namely, 
> with the sizes and signedness of the target system.  So, people should feel 
> free to check in
>  basis-library/config/c/$(TARGET_ARCH)-$(TARGET_OS)/c-types.sml
>
> The 'vision' is to develop a signature of operations that may be applied to C 
> integer types, which, like C, applies to types of any signedness.

I've exposed a few more coercions to and from SysWord.word in the 
INTEGER_EXTRA signature, which fixes most of the signedness issues.  I can 
compile x86-mingw, with the signedness if ino_t restored.  The remaining 
issue is that MLTON_RLIMIT exposes the rlim_t type as WORD, which is 
consistent with the POSIX specs, which state that rlim_t is an unsigned. 
However, the BSD variants (darwin, freebsd) purposefully define rlim_t as 
a signed integer, for compat. with legacy code that uses -1 for infinity.

Suggestions on how to pass rlim_t up to ML user code in a portable manner?