Implement bignum system library (subset)

[axic]

See ewasm/design#189

It would be useful for benchmarking.

@chfast for this we need to include intx. Can we include it as a cmake project or we need to submodule it?

[chfast]

Probably the best would be to release version 0.1 and add it to Hunter.

[axic]

Is that realistic tomorrow?

[chfast]

Realistic in 8 hours, so I'm not sure about tomorrow.