Check Encoder/Decoder symmetry

The current Encoder/Decoder properties directly refer to the implementation objects, rather than enumerated options as in Compression/Decompression properties.

This makes it difficult (if not impossible) to check symmetry. (e.g., this edge X encodes/decodes in symmetry, this edge Y is directed to a StreamVertex and thus encodes/decodes asymmetrically)

It'd be good to use enumerated options for Encoder/Decoder properties to enable such checks.