When using this package, `ucan.Token` reads better than `ucan.UCAN` to me.
Encourages the reader to think in terms of "tokens", from the "ucan" package.
Performed this refactoring with a few commands (and a few comment edits):
$ gofmt -r 'UCAN -> Token' -w .
$ gofmt -r 'NewAttenuatedUCAN -> NewAttenuatedToken' -w .
$ gofmt -r 'NewOriginUCAN -> NewOriginToken' -w .
$ gofmt -r 'UCANParser -> TokenParser' -w .
$ gofmt -r 'NewUCANParser -> NewTokenParser' -w .
$ gofmt -r 'UCANCtxKey -> TokenCtxKey' -w .
Changed example test "ExampleWalkthrough" to "Example" to make go vet happy
BREAKING CHANGE:
UCAN symbol is now "Token"
