Improve BitVecValue representation.

[ekiwi]

Since we never resize the underlying vector, we should be able to use a Box<[Word]> and thus reduce the size of a BitVecValue from 32 to 24 bytes. This means that we have to implement the SmallVec optimization ourselves. This may best be accomplished by making an enum like enum Data { U64(u64), U128(128), Big(Box<[Word]>) }. This way, we may be able to improve the performance when executing operations on values with less than 128 bits.