Avoid code duplication between keep-core and keep-tecdsa chain interfaces

[pdyraga]

We need to figure out how to integrate keep-core chain contract Go code generators and chain-specific interfaces into keep-tecdsa to avoid code duplication.

This card covers all the work required to remove code duplication and to refer to the same codebase.

[Shadowfiend]

Fwiw, I'm trying to solve the giant issues that keep-network/keep-core#491 represents first, then follow up by figuring out how to factor out the code generators, and finally how we can pull them into this and other repos. Mostly doing this because I expect 491 will result in significant changes to both the generated code and chain interface style, so integrating it here will also require some bonus work.

More to come as I discover exactly how much of a pain 491 will be (I've already determined the minimum pain level is “slapped in the face repeatedly with a trout”).