Arithmetic operations not implemented in terms of BigBit formats themselves

[kotauskas]

Currently, the only way to perform arithmetic operations on BigBit numbers to convert them to BigNumbers, add these and encode them back to the original BigBit format. As the reference implementation, this strongly needs an implementation of BigBit arithmetics without BigNumbers, since such a type is not even present in certain languages, namely Rust, which I'm porting this format to (I'm trying to keep the crate as a zero-dependency self-sustaining implementation, thus using any other format as a helper construct simply hurts the proof of concept).