Optimize CodedBufferWriter._copyInto to memcpy (#885)

dart2wasm currently can't optimize loops into memcpy, however `setRange`
methods have type tests to generate `array.copy` (Wasm's `memcpy`).

Replacing the loops in `CodedBufferWriter._copyInto` with `setRange` improves
an internal benchmark extracted from a real use case significantly in all
targets:

|                              | Before     | After      | Diff                |
|------------------------------|------------|------------|---------------------|
| AOT                          | 127,587 us | 95,634 us  | -31,953 us,  -25.0% |
| JIT                          | 106,880 us | 92,800 us  | -14,080 us,  -13.1% |
| dart2js -O4                  | 285,587 us | 262,222 us | -23,365 us,  -8.1%  |
| dart2wasm --omit-type-checks | 337,000 us | 236,100 us | -100,900 us, -29.9% |

protobuf/lib/src/protobuf/coded_buffer_writer.dart

@@ -421,22 +421,19 @@ class CodedBufferWriter {
 
   /// Copy bytes from the given typed data array into the output buffer.
   ///
-  /// Has a specialization for Uint8List for performance.
+  /// Has a specialization for [Uint8List] for performance.
   int _copyInto(Uint8List buffer, int pos, TypedData value) {
     if (value is Uint8List) {
       final len = value.length;
-      for (var j = 0; j < len; j++) {
-        buffer[pos++] = value[j];
-      }
-      return pos;
+      final end = pos + len;
+      buffer.setRange(pos, end, value);
+      return end;
     } else {
       final len = value.lengthInBytes;
-      final u8 = Uint8List.view(
-          value.buffer, value.offsetInBytes, value.lengthInBytes);
-      for (var j = 0; j < len; j++) {
-        buffer[pos++] = u8[j];
-      }
-      return pos;
+      final end = pos + len;
+      final u8 = Uint8List.view(value.buffer, value.offsetInBytes, len);
+      buffer.setRange(pos, end, u8);
+      return end;
     }
   }