Have `hmac` outputs raw binary data rather than hexits lowercase

feature
2

What if we added an optional fourth (bool) arg?

{{ hmac "sha256" "My Secret" "My Message" true | encoding.Base64Encode }}

Or, we could add a HexDecode method to the encoding namespace, which wouldn’t mess with the piping sequence:

{{ "My Message" | hmac "sha256" "My Secret" | encoding.HexDecode | encoding.Base64Encode }}

encoding.HexDecode and encoding.HexEncode might also be useful for color conversions. These were implemented (but not merged) by @moorereason in #7605. I suspect these would be used infrequently, so I’m not sure it makes sense to give them aliases (e.g., hexDecode).