Split into different modules

[DevSide]

bn.js is used by a lot of packages but some of them does not use all bn's features and are meant to be run in a browser where package size is important in terms of downloading and parsing time.

I suggest splitting into different modules, objects and operators extending bn, in order to get the optimizer tools removing dead code (tree-shaking).

What do you think ?

[indutny]

I don't mind!