Add parity-codec cargo feature.

[dandanlen]

This is useful if you want to use ethabi in a substrate-related project, for example abi-encoding eth contract calls on-chain.

[vkgnosis]

Since nothing in this crate changes with the feature, wouldn't it make more sense that you depend on ethereum-types/codec when you need the feature there rather than going through this crate?

[dandanlen]

I could, but then I need to add an explicit dependency on ethereum-types, which is currently only a transitive dependency via ethabi.

For example, say I want to use Address type from ethabi. I don't really care that it is re-exported from ethereum-types, I just want it to be compatible with the ethabi crate.

I suppose I agree in that this feature would make more sense / would be more complete if it also conditionally derived the Encode and Decode traits for Function, Param etc.

[vkgnosis]

Makes sense to me. Will give the PR some time in case other people have an opinion.

[gakonst]

lgtm

[nlordell]

LGTM

[gakonst]

If you merge this, it'd also help to release a new version of the crate!