优化了dump性能，新增若干数据采样器

Unity3D/3rdLib/Utf8Json/Attributes.cs

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+using System;
+using System.Collections.Generic;
+using System.Text;
+
+namespace Utf8Json
+{
+    [AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct | AttributeTargets.Interface | AttributeTargets.Field | AttributeTargets.Property, AllowMultiple = false, Inherited = true)]
+    public class JsonFormatterAttribute : Attribute
+    {
+        public Type FormatterType { get; private set; }
+        public object[] Arguments { get; private set; }
+
+        public JsonFormatterAttribute(Type formatterType)
+        {
+            this.FormatterType = formatterType;
+        }
+
+        public JsonFormatterAttribute(Type formatterType, params object[] arguments)
+        {
+            this.FormatterType = formatterType;
+            this.Arguments = arguments;
+        }
+    }
+
+    [AttributeUsage(AttributeTargets.Constructor, AllowMultiple = false, Inherited = true)]
+    public class SerializationConstructorAttribute : Attribute
+    {
+
+    }
+}

Unity3D/3rdLib/Utf8Json/Formatters/AnonymousFormatter.cs

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+using System;
+
+namespace Utf8Json.Formatters
+{
+    public sealed class AnonymousFormatter<T> : IJsonFormatter<T>
+    {
+        readonly JsonSerializeAction<T> serialize;
+        readonly JsonDeserializeFunc<T> deserialize;
+
+        public AnonymousFormatter(JsonSerializeAction<T> serialize, JsonDeserializeFunc<T> deserialize)
+        {
+            this.serialize = serialize;
+            this.deserialize = deserialize;
+        }
+
+        public void Serialize(ref JsonWriter writer, T value, IJsonFormatterResolver formatterResolver)
+        {
+            if (serialize == null) throw new InvalidOperationException(this.GetType().Name + " does not support Serialize.");
+            serialize(ref writer, value, formatterResolver);
+        }
+
+        public T Deserialize(ref JsonReader reader, IJsonFormatterResolver formatterResolver)
+        {
+            if (deserialize == null) throw new InvalidOperationException(this.GetType().Name + " does not support Deserialize.");
+            return deserialize(ref reader, formatterResolver);
+        }
+    }
+}