amortize hasher initialization for Sum256/Sum512

As expected, the performance increase is only noticeable for
small inputs:

name            old time/op    new time/op    delta
Sum256/64-4        343ns ± 4%     243ns ± 6%  -28.96%  (p=0.008 n=5+5)
Sum256/1024-4     2.62µs ± 1%    2.50µs ± 2%   -4.57%  (p=0.008 n=5+5)
Sum256/65536-4     175µs ± 1%     170µs ± 4%     ~     (p=0.190 n=4+5)

Co-authored-by: Dmitry Yu Okunev <xaionaro@dx.center>

blake3.go

@@ -406,9 +406,13 @@ func New(size int, key []byte) *Hasher {
 	return newHasher(keyWords, flagKeyedHash, size)
 }
 
+// Sum256 and Sum512 always use the same hasher state, so we can save some time
+// when hashing small inputs by constructing the hasher ahead of time.
+var defaultHasher = newHasher(iv, 0, 0)
+
 // Sum256 returns the unkeyed BLAKE3 hash of b, truncated to 256 bits.
 func Sum256(b []byte) (out [32]byte) {
-	h := newHasher(iv, 0, 0)
+	h := *defaultHasher
 	h.Write(b)
 	h.XOF().Read(out[:])
 	return
@@ -416,7 +420,7 @@ func Sum256(b []byte) (out [32]byte) {
 
 // Sum512 returns the unkeyed BLAKE3 hash of b, truncated to 512 bits.
 func Sum512(b []byte) (out [64]byte) {
-	h := newHasher(iv, 0, 0)
+	h := *defaultHasher
 	h.Write(b)
 	h.XOF().Read(out[:])
 	return

blake3_test.go

@@ -195,11 +195,27 @@ func BenchmarkWrite(b *testing.B) {
 }
 
 func BenchmarkSum256(b *testing.B) {
-	b.ReportAllocs()
-	buf := make([]byte, 1024)
-	for i := 0; i < b.N; i++ {
-		blake3.Sum256(buf)
-	}
+	b.Run("64", func(b *testing.B) {
+		b.ReportAllocs()
+		buf := make([]byte, 64)
+		for i := 0; i < b.N; i++ {
+			blake3.Sum256(buf)
+		}
+	})
+	b.Run("1024", func(b *testing.B) {
+		b.ReportAllocs()
+		buf := make([]byte, 1024)
+		for i := 0; i < b.N; i++ {
+			blake3.Sum256(buf)
+		}
+	})
+	b.Run("65536", func(b *testing.B) {
+		b.ReportAllocs()
+		buf := make([]byte, 65536)
+		for i := 0; i < b.N; i++ {
+			blake3.Sum256(buf)
+		}
+	})
 }
 
 func BenchmarkXOF(b *testing.B) {