Rust: Encoding Panic with valid schema and input

After a recent bug regarding Rust Avro's schema ref was fixed to help preserve the parsing form of schema (https://issues.apache.org/jira/browse/AVRO-3433). This exposed an issue where named schemas defined with the rules of the spec (depth first, left to right), are not available to the encoding workflow because it indexes schemas as it traverses the data. This is a problem for the common use case of defining a record within an optional field and it being encoded as Null, and not having that type then available during encoding for the next field. 

This was patched for level + 1 schema definitions but was not solved within the fullness of the spec