hashBase is ambiguous, since it's not about the digital bases, but about
the format of hashes. Base16, Base32 and Base64 are all character maps
for binary encoding.
Rename the enum Base to HashFormat.
Rename variables of type HashFormat from [hash]Base to hashFormat,
including CmdHashBase::hashFormat and CmdToBase::hashFormat.
Instead, `Hash` uses `std::optional<HashType>`. In the future, we may
also make `Hash` itself require a known hash type, encoraging people to
use `std::optional<Hash>` instead.
Jonas Chevalier