utilities for schema import-from/export-to yaml.

SS.loadSchemaFromYaml will be deprecated in 0.8 and removed in 0.8+1. Moving forward, we should have a set of utilities in contrib that maintain the ability to set a schema in yaml and then load it on an empty node.

If at some point, the higher level clients make it easy to modify the schema, or the CLI becomes a real tool, we probably won't need this.