Integrate go-varsig and go-did-it

- go-varsig provides a varsig V1 implementation
- go-did-it provides a complete and extensible DID implementation

token/delegation/schema.go

@@ -14,10 +14,10 @@ import (
 	"github.com/ucan-wg/go-ucan/token/internal/envelope"
 )
 
-// [Tag] is the string used as a key within the SigPayload that identifies
+// Tag is the string used as a key within the SigPayload that identifies
 // that the TokenPayload is a delegation.
 //
-// [Tag]: https://github.com/ucan-wg/delegation/tree/v1_ipld#type-tag
+// See: https://github.com/ucan-wg/delegation/tree/v1_ipld#type-tag
 const Tag = "ucan/dlg@1.0.0-rc.1"
 
 // TODO: update the above Tag URL once the delegation specification is merged.