Add a hash prefix for ARCs-related hashes

This is to allow ARCs (github.com/algorandfoundation/ARCs)
to have their own hash prefix without risk of collision.

Author: fabrice102

protocol/hash.go

@@ -23,7 +23,15 @@ type HashID string
 // Hash IDs for specific object types, in lexicographic order.
 // Hash IDs must be PREFIX-FREE (no hash ID is a prefix of another).
 const (
-	AppIndex          HashID = "appID"
+	AppIndex HashID = "appID"
+
+	// ARCReserved is used to reserve prefixes starting with `arc` to
+	// ARCs-related hashes https://github.com/algorandfoundation/ARCs
+	// The prefix for ARC-XXXX should start with:
+	// "arcXXXX" (where "XXXX" is the 0-padded number of the ARC)
+	// For example ARC-0003 can use any prefix starting with "arc0003"
+	ARCReserved HashID = "arc"
+
 	AuctionBid        HashID = "aB"
 	AuctionDeposit    HashID = "aD"
 	AuctionOutcomes   HashID = "aO"