WASM runtime update

[NikVolf]

• WASM contracts will be able to utilize various blockchain runtime externs (gas_limit, difficulty, coinbase, etc..)
• corresponding happy test
• also updates to use latest parity-wasm and corresponding refactoring

[svyatonik]

LGTM, but I would replace exact gas_left checks in tests with something like assert!(gas_left < params.gas) or assert!(params.gas - gas_left < 5000).
Not sure that exact value is important - imho it depends on too many factors like compiler version (next version may generate slightly different wasm code), compiler flags (something like optimization/no optimization, etc), code itself.

[NikVolf]

@svyatonik
I want gas price to match exactly when we don't change wasm itself, it's consensus-critical issue

[rphmeier]

@NikVolf I agree. Related: what made the gas costs for examples smaller? I was worried that they aren't remaining consistent.

[NikVolf]

@rphmeier
oh we change the pwasm-std currently a lot, so the way wasm produced in tests repository will change from time to time

I am not sure about what exactly caused this particular change though, nor it does matter much