ENH: add refPtr release() method

- releases ownership of the pointer. A no-op (and returns nullptr)
  for references.

  Naming consistent with unique_ptr and autoPtr.

DOC: adjust wording for memory-related classes

- add is_const() method for tmp, refPtr.

  Drop (ununsed and confusing looking) isTmp method from refPtr
  in favour of is_pointer() or movable() checks

ENH: noexcept for some pTraits methods, remove redundant 'inline'

- test for const first for tmp/refPtr (simpler logic)