This allows users of a valid Digest to know it has a minimum of 2
characters in the hash part for use when sharding.
This is a reasonable restriction as the hash part is a SHA256 hash which
is 64 characters long, which is the common hash used. There is no
anticipation of using a hash with less than 2 characters.
Also, add MustParseDigest.
Also, replace Digest.Type with Digest.Split for getting both the type
and hash parts together, which is most the common case when asking for
either.
