Need type encoders/decoders for built-in data types

[b3m2a1]

Encoders and decoders should be written for important data types (SparseArray, Graph, Region, etc.) so that users can build off of these.

[b3m2a1]

Encoders for PackedArray, Image, RawArray, and SparseArray have been written. Still need Graph, Region, MeshRegion, etc.