[MLton] fold on the wiki

[Stephen Weeks]

I put up a page on the wiki to collect together our knowledge about
fold and to serve as exposition for those new to the technique.

  http://mlton.org/Fold

I added some new pages with examples of Fold

  http://mlton.org/ArrayLiteral
  http://mlton.org/Fold01N
  http://mlton.org/NumericLiteral
  http://mlton.org/VariableArityPolymorphism

I changed (hopefully simplified) some existing pages to use fold.

  http://mlton.org/FunctionalRecordUpdate
  http://mlton.org/OptionalArguments
  http://mlton.org/Printf

I tried to get most of the ideas that had floated around on the
mailing list, but there is a lot of new stuff, so there are surely
errors and omissions.  Please make corrections and improvements
directly to the wiki or suggest them on this list.  Thanks.