Add a fast path for string inputs

```
                  │ master.bench │         perf-string.bench          │
                  │    sec/op    │   sec/op     vs base               │
String/default-10    86.98n ± 3%   77.57n ± 1%  -10.82% (p=0.002 n=6)
String/xxhash-10     40.10n ± 0%   34.82n ± 4%  -13.19% (p=0.002 n=6)
geomean              59.06n        51.97n       -12.01%

                  │ master.bench │         perf-string.bench          │
                  │     B/op     │    B/op     vs base                │
String/default-10     56.00 ± 0%   56.00 ± 0%       ~ (p=1.000 n=6) ¹
String/xxhash-10      16.00 ± 0%   16.00 ± 0%       ~ (p=1.000 n=6) ¹
geomean               29.93        29.93       +0.00%
¹ all samples are equal

                  │ master.bench │         perf-string.bench          │
                  │  allocs/op   │ allocs/op   vs base                │
String/default-10     3.000 ± 0%   3.000 ± 0%       ~ (p=1.000 n=6) ¹
String/xxhash-10      1.000 ± 0%   1.000 ± 0%       ~ (p=1.000 n=6) ¹
geomean               1.732        1.732       +0.00%
```

hashstructure.go

@@ -91,6 +91,11 @@ func Hash(v interface{}, opts *HashOptions) (uint64, error) {
 	// Reset the hash
 	opts.Hasher.Reset()
 
+	// Fast path for strings.
+	if s, ok := v.(string); ok {
+		return hashString(opts.Hasher, s)
+	}
+
 	// Create our walker and walk the structure
 	w := &walker{
 		h:               opts.Hasher,
@@ -130,6 +135,21 @@ func (w *walker) hashDirect(v any) (uint64, error) {
 	return w.h.Sum64(), err
 }
 
+// A direct hash calculation used for strings.
+func (w *walker) hashString(s string) (uint64, error) {
+	return hashString(w.h, s)
+}
+
+// A direct hash calculation used for strings.
+func hashString(h hash.Hash64, s string) (uint64, error) {
+	h.Reset()
+
+	// io.WriteString uses io.StringWriter if it exists, which is
+	// implemented by e.g. github.com/cespare/xxhash.
+	_, err := io.WriteString(h, s)
+	return h.Sum64(), err
+}
+
 func (w *walker) visit(v reflect.Value, opts *visitOpts) (uint64, error) {
 	t := reflect.TypeOf(0)
 
@@ -405,14 +425,7 @@ func (w *walker) visit(v reflect.Value, opts *visitOpts) (uint64, error) {
 		return h, nil
 
 	case reflect.String:
-		// Directly hash
-		w.h.Reset()
-
-		// io.WriteString uses io.StringWriter if it exists, which is
-		// implemented by e.g. github.com/cespare/xxhash.
-		_, err := io.WriteString(w.h, v.String())
-		return w.h.Sum64(), err
-
+		return w.hashString(v.String())
 	default:
 		return 0, fmt.Errorf("unknown kind to hash: %s", k)
 	}