Datastream (de)serialization

[toonsevrin]

Add a library to (de)serialize all methods. It should perhaps be done in a seperate project so it can be used again in the javascript library.

Some resources:

• https://github.com/EOSIO/eos/blob/master/contracts/eosiolib/datastream.hpp
• https://github.com/EOSArgentina/gameoflife-ts/blob/master/src/eoslib/datastream.ts

I haven't started on this so feel free to assign it to yourself or a team member!

[toonsevrin]

Should be implemented here: https://github.com/eos-assemblyscript/eos-assemblyscript-serializer