Contract runtime: better big ints

[vgrichina]

https://github.com/MaxGraey/bignum.wasm unfortunately is still lacking in features and implemented in a way where each int length requires complete rewrite.

We should instead port or implement variable length (multiple of 32-bit) integers and create thin wrapper classes with limitation (including overflow/underflow protection).

See http://peterolson.github.io/BigInteger.js/benchmark/ for libraries that can potentially be ported.