[Java][FlexBuffers] Optimize Map access (#5735)

The original implementation of map access is very naive:
- Encode String to UTF8 byte[]
- Creates a new KeyVector
- Performs a binary search to find the key
- return value

So every access to the Map there was useless allocations of Keys and KeyVector
and complete encoding of the search key, which for most comparisons would be wasteful.

This changes completely removes the use of KeyVector and compute the key
positions on the spot. Besides that, it compares keys codepoint-by-codepoint,
avoiding unnecessary allocations and reducing encoding for most cases.

Some benchmarks result in a 2.75x speedup.

tests/JavaTest.java

@@ -12,10 +12,12 @@ import com.google.flatbuffers.FlexBuffers;
 import com.google.flatbuffers.FlexBuffersBuilder;
 import com.google.flatbuffers.StringVector;
 import com.google.flatbuffers.UnionVector;
+
 import com.google.flatbuffers.FlexBuffers.FlexBufferException;
 import com.google.flatbuffers.FlexBuffers.Reference;
 import com.google.flatbuffers.FlexBuffers.Vector;
 import com.google.flatbuffers.ArrayReadWriteBuf;
+import com.google.flatbuffers.FlexBuffers.KeyVector;
 
 import java.io.*;
 import java.math.BigInteger;
@@ -1024,6 +1026,41 @@ class JavaTest {
             // It should throw exception
         }
     }
+    
+    public static void testFlexBuffersUtf8Map() {
+        FlexBuffersBuilder builder = new FlexBuffersBuilder(ByteBuffer.allocate(512),
+                FlexBuffersBuilder.BUILDER_FLAG_SHARE_KEYS_AND_STRINGS);
+
+        String key0 = "😨 face1";
+        String key1 = "😩 face2";
+        String key2 = "😨 face3";
+        String key3 = "trademark ®";
+        String key4 = "€ euro";
+        String utf8keys[] = { "😨 face1", "😩 face2", "😨 face3", "trademark ®", "€ euro"};
+
+        int map = builder.startMap();
+
+        for (int i=0; i< utf8keys.length; i++) {
+            builder.putString(utf8keys[i], utf8keys[i]);  // Testing key and string reuse.
+        }
+        builder.endMap(null, map);
+        builder.finish();
+
+        FlexBuffers.Map m = FlexBuffers.getRoot(builder.getBuffer()).asMap();
+
+        TestEq(m.size(), 5);
+
+        KeyVector kv = m.keys();
+        for (int i=0; i< utf8keys.length; i++) {
+            TestEq(kv.get(i).toString(), m.get(i).asString());
+        }
+
+        TestEq(m.get(key0).asString(), utf8keys[0]);
+        TestEq(m.get(key1).asString(), utf8keys[1]);
+        TestEq(m.get(key2).asString(), utf8keys[2]);
+        TestEq(m.get(key3).asString(), utf8keys[3]);
+        TestEq(m.get(key4).asString(), utf8keys[4]);
+    }
 
     public static void TestFlexBuffers() {
         testSingleElementByte();
@@ -1047,6 +1084,7 @@ class JavaTest {
         testFlexBufferVectorStrings();
         testDeprecatedTypedVectorString();
         testBuilderGrowth();
+        testFlexBuffersUtf8Map();
     }
 
     static void TestVectorOfBytes() {