Cmm arithmetic optimisations

Some optimisations on Cmm expressions involving tagging or constants. Here's a (best-case) example:

    let int_of_digits a b c = 
      100 * (Char.code a - Char.code '0') + 
       10 * (Char.code b - Char.code '0') +
        1 * (Char.code c - Char.code '0')

Trunk compiles this to:

     (+
       (+ (+ (* 200 (>>s (+ a/1020 -96) 1)) (* 20 (>>s (+ b/1021 -96) 1)))
         (* 2 (>>s (+ c/1022 -96) 1)))
       1)

This branch compiles this to:

    (+ (+ (+ ( * a/1020 100) ( * b/1021 10)) c/1022) -10766)

Floating all of the constant operations, tags, etc. out of arithmetic means they can be merged into one addition.
`Sys.word_size` should be used instead of this.