Made URI and IRI conform to the LinkOrUri interface

Added a convenience String method

activitypub/uri.go

@@ -7,3 +7,17 @@ type (
 	// URI is a Uniform Resource Identifier (URI) RFC3986
 	URI string
 )
+
+func (u URI) String() string {
+	return string(u)
+}
+func (i IRI) String() string {
+	return string(i)
+}
+
+func (u URI) GetLink() URI {
+	return u
+}
+func (i IRI) GetLink() URI {
+	return URI(i)
+}