The exported StringSet type is not necessary for the current use case of
validating issues and audiences. The exported fields on VerifyOptions have been
changed to require string slices. The collections package has been removed and
the StringSet has been moved to the token package, where it is used.
Signed-off-by: Stephen J Day <stephen.day@docker.com>
As part of the efforts to break up the common package before disaster strikes,
a new collections package has been created. More may belong there but for now,
it only includes an implementation of StringSet.
Signed-off-by: Stephen J Day <stephen.day@docker.com>