Clean-up `felt` Marshal/Unmarshal methods after full migration to json/v2

[RafaelGranza]

Felt and Slice should have only the following json Interfaces:

• MarshalText
• AppendText
• UnmarshalText
• UnmarshalJSONFrom
• MarshalJSONTo

This way we keep only v2 optimized ones.

Nowadays, the v1 is a mess, a lot of asymmetry surges when trying to optimize for CPU usage.
ex: MarshalText and UnmashalJSON is the fastest pair for encoding/deconding.

This asymmetry is fixed on v2.