This PR does a few things:

• Adds Reduce and Count functions to slicesextended.
• Introduces ParseError to bufcas. An error of type *bufcas.ParseError is returned from bufcas.Parse.* functions on unsuccessful parsing. This will allow us to understand the nature of an error returned from the new ModuleKey.Digest(), and handle situations such as Ignore b1/b3 buf.lock digests only #2111. Instead of blanking out the error from a buf.lock file, we can instead return an error saying we did not understand the DigestType, and handle that accordingly in places where we don't support it.
• Adds some extra functions to bufcas with respect to conversion to/from repeated Protobuf types.
• Updates dependencies as well.