Add from_token feature to allow decoding to rust type.

[tjsharp1]

For the WASM indexers, it will be useful to convert from an ABI-encoded byte blob back into the rust type defined in the contract.
Currently, we can go from a struct into ABI encoded bytes like this: my_struct.into_token(), the WASM indexers will also need to be able to do something like let struct = MyStruct::from_token(decoded_tokens).