[MLton] Bug in LargeInt.scan

[Stephen Weeks]

> Note that strings such as "0xg" and "0x 123" are scanned as SOME(0),  
> even using a hexadecimal radix.
> 
> So, that patch still isn't correct.
> A better patch is attached.

Patch cleaned up and committed.  Thanks.

Here's the definition of toHexR that I went with.

	 fun toHexR (cread: (char, 'a) reader, uread: (bigInt, 'a) reader) 
	    s =
            case Reader.reader2 cread s of
               NONE => NONE
             | SOME ((c1, c2), s') =>
                  if c1 = #"0" andalso (c2 = #"x" orelse c2 = #"X") then
                     case uread s' of 
                        NONE => uread s (* ? *)
                      | SOME x => SOME x
                  else uread s

I was wondering if in the case marked (* ? *) I could simply return
SOME zero.