From Spark: Add support for 128-bit bitvectors

This entails creating the modules for conversions between these and smaller bitvectors, as well as to the larger (and newly created) 256-bit bitvector for verifying absence of overflows on operations over 128-bit bitvectors.

Change-Id: I6089b63b7c37c6cc6b00ab13795e2edeb8b048df