[MLton] Char.scan

[Matthew Fluet]

MLton currently disagrees with SML/NJ on Char.scan when encountering a ".
There was some discussion on the basis-library list, which ended with
Steve arguing:

> Progress.  Here is my current understanding of the proposed changes
> and clarifications.  I've added a couple of logical (I hope)
> consequences regarding Char.scan.
>
> 1. Char.scan returns NONE upon double quote.
> 2. String.scan stops upon double quote, and returns the output to that
>    point.
> 3. String.scan consumes format sequences.
> 4. Char.scan consumes format sequences adjacent to a valid character.
>    I mention this because of pathological character constants like
>    #"a\ \", #"\ \a", #"\ \a\ \", and #"\ \\ \a".
> 5. Char.scan does not consume format sequences if it is unable to
>    produce a character.  Consider the invalid character constant
>    #"\ \".
> 6. String.scan never returns NONE.

MLton currently disagrees with 1; I don't know about the other items.  In
any case, this was (the only) show-stopper in compiling the ML-Doc tools
with MLton.

(There were a couple of or-patterns and vector literals that were trivial
to change.  It was also a pain changing all the .cm files to not use CM
anchors; unfortunately, I can't figure out a way of configuring SML/NJ so
that CM.Graph.graph on "$/smlnj-lib.cm" gives useful results.  The problem
is that all the anchored stuff is stabilized, so should be independent of
the sources.  I got around it by replicating (part of) the SML/NJ source
tree, and replacing all the anchored references (in both the SML/NJ and
ML-Doc sources) with relative references.  Then cmcat did it's thing.)