bn128 no-std version

[mersinvald]

From evm-rs created by mersinvald : etclabscore/evm-rs#26

Currently bn128 precompiled contract cannot be compiled for the no-std targets due to the rustc-serialize dependency.

There's a Serde version of bn crate at https://github.com/etclabscore/bn, but it's currently not compatible with the implementation in std/precompiled/bn128.