Config/Preset parsing is fixed on SSZ_NIF

[rodrigo-o]

In Elixir, the Chainspect is generated following a set of preset yamls but on the SSZ_NIF, config.rs (which is really the preset as mentioned in #604) sets all values as fixed Unsigned types in the config structs. This is error prone and it stays out of date when the yaml changed. We need to fix it.