Consistent function naming in Validator

[zac-garby]

In the validator, some functions end in question marks and some don't, where really they all should or they all shouldn't. For example, there's valid_hash/1 and valid_coinbase?/1. They both check if something's valid, but aren't consistent in their names.

[afbreilyn]

I think if the function only returns a boolean, then the question mark is appropriate. I'll follow this paradigm as I edit!