This PR improves the modes in predef.ml. Please review by commits:

• The first commit adds the interpretation of more mode annotation, to allow tests
• The second commit adds the jkind mutable_data.
• The third commit fixes up predef.ml in the following ways:
  • Add ~jkind to allow more mode crossing.
  • Add ~jkind_annotation when the corresponding ~jkind is not value
  • Add ~param_jkind to iarray, to require elements of iarray to be non-null.
  • Also some reformatting to avoid long lines.