Split out bn128 helpers into a separate package

[axic]

This is kind of a feature request.

Currently the precompiled contracts are implemented in ethcore/builtin. The bn128 ecadd/ecmul/ecpairing precompiles are implemented there as well, using the bn crate as the underlying library.

In ethereumjs-vm the same underlying library is used and part of the code is duplicated as a wrapper library rustbn.js.

Now additionally we are experimented with writing the precompiles as contracts in the ewasm project and as a result we need the very same wrapper, hence we have split out the above code from builtin.rs into a crate of its own: https://github.com/ewasm/ethereum-bn128.rs

It can now be used by both rustbn.js and the contract.

This works well, but it seems like a bad idea to maintain a fork and such would like to invite the maintainers to consider splitting out that code into a crate maintained in this org and used by the Parity client too.

Would that be considered? Are there are any reasons against it? Would you accept a PR to do this?

[NikVolf]

Hi axic!

Sure, we will accept the pr, no reason against it.

But the code in ethcore/builtin is merely deserialization and serialization of points and field members, all the work is done in bn itself, what do you want to split out exactly?

[axic]

If you can have a look at https://github.com/ewasm/ethereum-bn128.rs/blob/master/src/lib.rs it has the parts we need to be split: the (de)serialisation and the test cases.

I'd assume these internal helpers would then be called by the Builtin
implementation for the said precompiles.

The gas calculation could also go in there, but not necessarily.

[debris]

we just split builtins into it's own crate (#10850). for now, further splits are not planned