Add test that casting to an invalid type raises LoopyError.

Disallow casting a non-numeric subtype to a numeric type.

This allows invocation of pytest from an arbitrary directory to
succeed in running the test.

Fixes #99.

  constructor.
* Check that all instruction ids are unique on kernel creation.
* Add a uniquify_instruction_ids() transform.

This closes #89.

Among other things, passing fixed parameters can improve the accuracy
of temp shape inference, so that fallback can occur less often.

The motivation for this is to remove the temp_shape_fallback warnings
when constructing sumpy kernels.

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

This change also renames LazilyUnpicklingDictionary to LazyDict.