Replace the MD5 hashing of images with xxHash

Note that we only use this for change detection.

The previous implementation invoked `MD5FromReaderFast` that created a MD5 has from 8 64 bytes chunks in the file, which is obviously very fast. The new implementation creates the hash from the entire file and ... seems to be even more effective:

```
name          old time/op    new time/op    delta
HashImage-10    9.45µs ±21%   10.89µs ± 1%     ~     (p=0.343 n=4+4)

name          old alloc/op   new alloc/op   delta
HashImage-10      144B ± 0%        8B ± 0%  -94.44%  (p=0.029 n=4+4)

name          old allocs/op  new allocs/op  delta
HashImage-10      4.00 ± 0%      1.00 ± 0%  -75.00%  (p=0.029 n=4+4)
```

common/hashing/hashing_test.go

@@ -0,0 +1,79 @@
+// Copyright 2024 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package hashing
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/cespare/xxhash/v2"
+	qt "github.com/frankban/quicktest"
+)
+
+func TestXxHashFromReader(t *testing.T) {
+	c := qt.New(t)
+	s := "Hello World"
+	r := strings.NewReader(s)
+	got, size, err := XXHashFromReader(r)
+	c.Assert(err, qt.IsNil)
+	c.Assert(size, qt.Equals, int64(len(s)))
+	c.Assert(got, qt.Equals, uint64(7148569436472236994))
+}
+
+func TestXxHashFromString(t *testing.T) {
+	c := qt.New(t)
+	s := "Hello World"
+	got, err := XXHashFromString(s)
+	c.Assert(err, qt.IsNil)
+	c.Assert(got, qt.Equals, uint64(7148569436472236994))
+}
+
+func TestXxHashFromStringHexEncoded(t *testing.T) {
+	c := qt.New(t)
+	s := "The quick brown fox jumps over the lazy dog"
+	got := XxHashFromStringHexEncoded(s)
+	// Facit: https://asecuritysite.com/encryption/xxhash?val=The%20quick%20brown%20fox%20jumps%20over%20the%20lazy%20dog
+	c.Assert(got, qt.Equals, "0b242d361fda71bc")
+}
+
+func BenchmarkXXHashFromReader(b *testing.B) {
+	r := strings.NewReader("Hello World")
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		XXHashFromReader(r)
+		r.Seek(0, 0)
+	}
+}
+
+func BenchmarkXXHashFromString(b *testing.B) {
+	s := "Hello World"
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		XXHashFromString(s)
+	}
+}
+
+func BenchmarkXXHashFromStringHexEncoded(b *testing.B) {
+	s := "The quick brown fox jumps over the lazy dog"
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		XxHashFromStringHexEncoded(s)
+	}
+}
+
+func xxHashFromString(f string) uint64 {
+	h := xxhash.New()
+	h.WriteString(f)
+	return h.Sum64()
+}