Remove a closure in _FieldSet.hashCode (#633)

* Remove a closure in `_FieldSet.hashCode`

Improves HashCode benchmark up to 3%.

JIT before:

    HashCode(RunTime): 1981.9821782178217 us.
    HashCode(RunTime): 1974.7068114511353 us.
    HashCode(RunTime): 1970.9261083743843 us.

JIT after:

    HashCode(RunTime): 1899.034155597723 us.
    HashCode(RunTime): 1912.0917782026768 us.
    HashCode(RunTime): 1908.1782650142993 us.

Native AOT before:

    HashCode(RunTime): 5164.038659793814 us.
    HashCode(RunTime): 5177.984496124031 us.
    HashCode(RunTime): 5192.199481865285 us.

Native AOT after:

    HashCode(RunTime): 5027.246231155779 us.
    HashCode(RunTime): 5045.488664987405 us.
    HashCode(RunTime): 5134.379487179487 us.

JS before:

    HashCode(RunTime): 6944.444444444444 us.
    HashCode(RunTime): 6913.793103448276 us.
    HashCode(RunTime): 6927.335640138408 us.

JS after:

    HashCode(RunTime): 6846.41638225256 us.
    HashCode(RunTime): 6965.277777777777 us.
    HashCode(RunTime): 6920.415224913495 us.

Fixes #632

protobuf/lib/src/protobuf/field_set.dart

@@ -654,29 +654,6 @@ class _FieldSet {
     if (_hashCodesCanBeMemoized && _memoizedHashCode != null) {
       return _memoizedHashCode!;
     }
-    // Hashes the value of one field (recursively).
-    int hashField(int hash, FieldInfo fi, value) {
-      if (value is List && value.isEmpty) {
-        return hash; // It's either repeated or an empty byte array.
-      }
-
-      hash = _HashUtils._combine(hash, fi.tagNumber);
-      if (_isBytes(fi.type)) {
-        // Bytes are represented as a List<int> (Usually with byte-data).
-        // We special case that to match our equality semantics.
-        hash = _HashUtils._combine(hash, _HashUtils._hashObjects(value));
-      } else if (!_isEnum(fi.type)) {
-        hash = _HashUtils._combine(hash, value.hashCode);
-      } else if (fi.isRepeated) {
-        hash = _HashUtils._combine(
-            hash, _HashUtils._hashObjects(value.map((enm) => enm.value)));
-      } else {
-        ProtobufEnum enm = value;
-        hash = _HashUtils._combine(hash, enm.value);
-      }
-
-      return hash;
-    }
 
     // Hash with descriptor.
     var hash = _HashUtils._combine(0, _meta.hashCode);
@@ -686,7 +663,7 @@ class _FieldSet {
     for (final fi in _infosSortedByTag) {
       final value = values[fi.index!];
       if (value == null) continue;
-      hash = hashField(hash, fi, value);
+      hash = _hashField(hash, fi, value);
     }
 
     // Hash with extension fields.
@@ -695,7 +672,7 @@ class _FieldSet {
       final sortedByTagNumbers = _sorted(extensions._tagNumbers);
       for (final tagNumber in sortedByTagNumbers) {
         final fi = extensions._getInfoOrNull(tagNumber)!;
-        hash = hashField(hash, fi, extensions._getFieldOrNull(fi));
+        hash = _hashField(hash, fi, extensions._getFieldOrNull(fi));
       }
     }
 
@@ -710,6 +687,30 @@ class _FieldSet {
     return hash;
   }
 
+  // Hashes the value of one field (recursively).
+  static int _hashField(int hash, FieldInfo fi, value) {
+    if (value is List && value.isEmpty) {
+      return hash; // It's either repeated or an empty byte array.
+    }
+
+    hash = _HashUtils._combine(hash, fi.tagNumber);
+    if (_isBytes(fi.type)) {
+      // Bytes are represented as a List<int> (Usually with byte-data).
+      // We special case that to match our equality semantics.
+      hash = _HashUtils._combine(hash, _HashUtils._hashObjects(value));
+    } else if (!_isEnum(fi.type)) {
+      hash = _HashUtils._combine(hash, value.hashCode);
+    } else if (fi.isRepeated) {
+      hash = _HashUtils._combine(
+          hash, _HashUtils._hashObjects(value.map((enm) => enm.value)));
+    } else {
+      ProtobufEnum enm = value;
+      hash = _HashUtils._combine(hash, enm.value);
+    }
+
+    return hash;
+  }
+
   void writeString(StringBuffer out, String indent) {
     void renderValue(key, value) {
       if (value is GeneratedMessage) {