util/sha256x: rename Hash as Block512 (#5351)

Rename Hash as Block512 to indicate that this is a general-purpose
hash.Hash for any algorithm that operates on 512-bit block sizes.

While we rename the package as hashx in this commit,
a subsequent commit will move the sha256x package to hashx.
This is done separately to avoid confusing git.

Signed-off-by: Joe Tsai <joetsai@digital-static.net>

util/deephash/deephash.go

@@ -34,7 +34,7 @@ import (
 	"time"
 	"unsafe"
 
-	"tailscale.com/util/sha256x"
+	hashx "tailscale.com/util/sha256x"
 )
 
 // There is much overlap between the theory of serialization and hashing.
@@ -82,7 +82,7 @@ const scratchSize = 128
 // hasher is reusable state for hashing a value.
 // Get one via hasherPool.
 type hasher struct {
-	sha256x.Hash
+	hashx.Block512
 	scratch    [scratchSize]byte
 	visitStack visitStack
 }
@@ -111,6 +111,13 @@ func initSeed() {
 	seed = uint64(time.Now().UnixNano())
 }
 
+func (h *hasher) Reset() {
+	if h.Block512.Hash == nil {
+		h.Block512.Hash = sha256.New()
+	}
+	h.Block512.Reset()
+}
+
 func (h *hasher) sum() (s Sum) {
 	h.Sum(s.sum[:0])
 	return s