C message supports Longs now (array of two ints)

Also add a few more tests for sethandles of different types
Add conversions to/from int64 values

* run.c converts from Array to -> int64
* nb_handle_convert_to_var converts from int64 -> Array
* added C generator tests; changed them to use strings instead of
  stringifying.
Use nb_var_* to read/write 64-bit ints

This will make it easier to change the serialized format of 64-bit ints
if necessary. Also, the code that uses it is cleaner.