call class serialize/deserialize methods

[mehtaphysical]

Fixes #19

This allows for something like:

export class PublicKey extends Assignable {
    keyType: KeyType;
    data: Uint8Array;

    serialize(writer: BinaryWriter) {
        writer.writeU8(this.keyType);
        writer.writeFixedArray(this.data);
    }

    static deserialize(reader: BinaryReader): PublicKey {
        const keyType = reader.readU8();
        const size = KeyType.ED25519 ? 32 : 65;
        return new PublicKey({ keyType, data: reader.readFixedArray(size) });
    }
}

[ailisp]

nice cleanup!

[ailisp]

I would prefer obj.borshSerialize/obj.borshDeserialize for a less chance of name collision

[ailisp]

Code change looks good to me, @volovyk-s do you think the API change makes sense?

[volovyks]

Looks good overall. Let's add simple unit tests and merge this PR.