deprecates api#Name.decoded and api#Name.encoded

Presence of both decoded/encoded and decodedName/encodedName has always
baffled me, as one of those method groups is clearly redundant, and
this pull request presents a great opportunity to address this by
deprecating one of the groups.

After some deliberation, I’ve chosen decoded/encoded as the deprecation
target, because its derivation from decodedName/encodedName is easier
than the other way around.