Combine step and definition structures

This change combines the Step and StepDefinition structures.

In order to combine these structures, we also needed to generate the Go types from the JSON Schema (instead of the other way around). The schema generation library we were using couldn't hand recursion. And the schema is a better single source-of-truth anyway, so we want to write that by hand.

The first commit creates the new schema from scratch in the schema/v0 folder. The second commit replaces schema/v1 with v0 so you can see how they relate to each other. You can review it commit-wise for all together, whatever is the most clear for you.

Note this replaces the schema without breaking changes. So all existing tests pass. However it also allows us to expand easily into a few additional valid usecases. See the issue for details.

Fixes #103 (closed)