Encoder for Index a.

Only encoding fields required to recreate a working index.

The following fields are not saved as they are restored via the provided Config on fromString.

• ref
• fields
• transformFactories
• filterFactories

The following fields are not saved because they are an acceleration model, decoder needs to set it on fromString.

• corpusTokensIndex

The following fields are not saved because they are caches and are cached as operationg requires

• transforms
• filters
• idfCache

Do not need an (a -> Encode.Value) because a is a document type and that is never encoded from an Index.

Encode CodecIndexRecord.