Function mutation_from_json 

pub fn mutation_from_json(v: &Value) -> Result<Mutation, String>

Parse one change-source ingest row change (see mutation_to_json) into a Mutation. Err carries a human reason for the malformed cell/op (the caller maps it to the wire error). Only add/remove/edit exist today; the partial update op is deferred with the Postgres connector (CHANGE-SOURCE-DESIGN.md §3, §8.4).