lib/model, lib/weakhash: Hash using adler32, add heuristic in puller

Adler32 is much faster, and the heuristic avoid the obvious cases where it
will not help.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3872

vendor/github.com/chmduquesne/rollinghash/rollinghash.go

@@ -0,0 +1,40 @@
+/*
+
+Package rollinghash implements rolling versions of some hashes
+
+*/
+package rollinghash
+
+import "hash"
+
+type Roller interface {
+	// Roll updates the hash of a rolling window from the entering byte.
+	// A copy of the window is internally kept from the last Write().
+	// This copy is updated along with the internal state of the checksum
+	// in order to determine the new hash very quickly.
+	Roll(b byte)
+}
+
+// rollinghash.Hash extends hash.Hash by adding the method Roll. A
+// rollinghash.Hash can be updated byte by byte, by specifying which byte
+// enters the window.
+type Hash interface {
+	hash.Hash
+	Roller
+}
+
+// rollinghash.Hash32 extends hash.Hash by adding the method Roll. A
+// rollinghash.Hash32 can be updated byte by byte, by specifying which
+// byte enters the window.
+type Hash32 interface {
+	hash.Hash32
+	Roller
+}
+
+// rollinghash.Hash64 extends hash.Hash by adding the method Roll. A
+// rollinghash.Hash64 can be updated byte by byte, by specifying which
+// byte enters the window.
+type Hash64 interface {
+	hash.Hash64
+	Roller
+}