embeddable implementation of contract reader

[EasterTheBunny]

When updates are made to a core interface and committed to common, builds break on core or relay repos due to not matching interfaces. This change proposes using 'unimplemented' versions of an interface to embed in downstream implementations to allow interface additions to be added without breaking downstream builds.

Usage:

type ImplementedContractReader struct {
    types.UnimplementedContractReader
}

// ... actual interface implementation below