Don't use json.marshal when calculating roots

[GalRogozinski]

Description

Currently Controller uses it which is bad

Consider using ssz here, as we use it mostly around the project, IMO we should stick to one deterministic serialization format, for example imagine a different language's json marshal will results in different order or some char (like ' instead of "), it will results in different hash.

Originally posted by @y0sher in #310 (comment)

[GalRogozinski]

I think for controller it is just used for tests