fix: use Buffer.from for stringification where available (#36)

Before:

```
Uint8Arrays.fromString x 1,203,638 ops/sec ±1.53% (85 runs sampled)
Uint8Arrays.toString x 376,183 ops/sec ±2.20% (74 runs sampled)
```

After:

```
Uint8Arrays.fromString x 2,051,032 ops/sec ±0.54% (88 runs sampled)
Uint8Arrays.toString x 2,912,464 ops/sec ±0.38% (91 runs sampled)
```

src/from-string.js

@@ -22,6 +22,10 @@ export function fromString (string, encoding = 'utf8') {
     throw new Error(`Unsupported encoding "${encoding}"`)
   }
 
+  if ((encoding === 'utf8' || encoding === 'utf-8') && globalThis.Buffer != null && globalThis.Buffer.from != null) {
+    return globalThis.Buffer.from(string, 'utf8')
+  }
+
   // add multibase prefix
   return base.decoder.decode(`${base.prefix}${string}`)
 }

src/to-string.js

@@ -22,6 +22,10 @@ export function toString (array, encoding = 'utf8') {
     throw new Error(`Unsupported encoding "${encoding}"`)
   }
 
+  if ((encoding === 'utf8' || encoding === 'utf-8') && globalThis.Buffer != null && globalThis.Buffer.from != null) {
+    return globalThis.Buffer.from(array.buffer, array.byteOffset, array.byteLength).toString('utf8')
+  }
+
   // strip multibase prefix
   return base.encoder.encode(array).substring(1)
 }