Wide integer types and fixed point arithmetics

[MaxGraey]

Hi @lrettig. I know some primitive methods for u128/u256 types provided in this project. But I think it not optimal in some parts and anyway required high level class-based wrapper on AssemblyScript side.

I started this isomorphic library for EOS/ewasm/parity contracts couple of month ago. And I need integration tests for each of the target platforms. Unfortunately I'm not so good about ewasm that it would be quick and right to write integrate tests. So if you were interested in helping with that, I would appreciate it very much!