They didn't even have public types, which made them pretty mysterious.

Give them types and reference the Decoder, which uses them.

Also, refer them qualified by their package name in the examples, as
we usually do in example*.go files, which usually use package foo_test
specifically so we can show the package names along with the symbols.

Change-Id: I50ebbbf43778c1627bfa526f8824f52c7953454f
Reviewed-on: https://go-review.googlesource.com/127663Reviewed-by: Bryan C. Mills <bcmills@google.com>