ENH: adjust HashTable iterator access

- previously allowed mapped value modification with a const iterator.
  However, this can lead to incorrect access patterns.

  Now tie the constness of the mapped value to that of the iterator.

- Remove deprecated iterator object() method.
  Was only used internally and was superseded by the val() method
  in an earlier version.

ENH: change isPointer to isPointerLike

- can distinguish between real pointers and wrapped pointers