Start tidying up the runtime:
- document most of the "size" and "length" parameters and global variables,
  specifying bytes or words. Change some of them from bytes to words.
- start getting rid of "char *" and use sensible types instead.
- other small changes


git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@15647 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02