Add supports of conversion between string and enum

[yestyle]

To support converting from &str to Enum and converting from Enum to String, add from_str() and implement Display for trait Enum respectively.

[stepancheg]

Merged with changes:

• no Display (I'm not 100% sure it should be)
• use string value directly without stringify! call
• use proto enum name instead of rust name in match (probably it is the same everywhere, but safer this way)

Thanks!