[External Codegen] Constant tensors in c-codegen

@trevor-m Thanks for confirming. I can’t talk about specific, but let’s just say any cpp serialization lib should be able to serialize/deserialize structs into a binary blob, and I am just using one of them.

Note that I’m not serializing Relay subgraph as it is, but some structs that get converted from Relay and amenable for serialization by a third party lib.