limit check insertion

[Henry Cejtin]

Yes,  the whole thing is pretty tricky to get type-safe.  Note, even ignoring
the relative pointers, once we go to arrays which don't have an  extra  level
of  indirection,  there will be a lot more multiplies by funny constants (the
size of an array element).  Thus there will be more of a  win  just  exposing
the  computation  of  the offset.  I.e., even if I can't keep around pointers
into the middle of an object, I can still keep around  the  offset  from  the
start  of  the array (as a word) and eliminate the multiplies using additions
of the size to induction variables.  This is a pretty big win.