Improve BitVecValue representation.

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.