This reverts merge request !297

Switching to *None* is not ideal, because *None* is already used in
temp_var_type to represent no value.

Hence, this change introduces an "optional" object wrapper to loopy
and uses it to represent the absence of a temp_var_type in an
assignment instruction.

Allow lang version setting with LOOPY_KERNEL_LANGUAGE_VERSION global in make_kernel caller's global space

This changes implements a LazyList and
LazyListWithEqAndPersistentHashing data structure. These data
structures lazily unpickle their contents.

This change also renames LazilyUnpicklingDictionary to LazyDict.

and __setstate__() doesn't run if the state is False.

Ensure that we always return a non-false state (another version bump
here).

This adds a tool for computing the iname nest efficiently, for printing.

Closes #5