Improve WriteString perf with SIMD
diff --git a/csharp/src/Google.Protobuf.Test/Buffers/ArrayBufferWriter.cs b/csharp/src/Google.Protobuf.Test/Buffers/ArrayBufferWriter.cs
index 5b9913b..a36a951 100644
--- a/csharp/src/Google.Protobuf.Test/Buffers/ArrayBufferWriter.cs
+++ b/csharp/src/Google.Protobuf.Test/Buffers/ArrayBufferWriter.cs
@@ -42,7 +42,7 @@
     /// ArrayBufferWriter is originally from corefx, and has been contributed to Protobuf
     /// https://github.com/dotnet/runtime/blob/071da4c41aa808c949a773b92dca6f88de9d11f3/src/libraries/Common/src/System/Buffers/ArrayBufferWriter.cs
     /// </summary>
-    internal sealed class ArrayBufferWriter<T> : IBufferWriter<T>
+    internal sealed class TestArrayBufferWriter<T> : IBufferWriter<T>
     {
         private T[] _buffer;
         private int _index;
@@ -50,10 +50,10 @@
         private const int DefaultInitialBufferSize = 256;
 
         /// <summary>
-        /// Creates an instance of an <see cref="ArrayBufferWriter{T}"/>, in which data can be written to,
+        /// Creates an instance of an <see cref="TestArrayBufferWriter{T}"/>, in which data can be written to,
         /// with the default initial capacity.
         /// </summary>
-        public ArrayBufferWriter()
+        public TestArrayBufferWriter()
         {
             _buffer = new T[0];
             _index = 0;
@@ -66,14 +66,14 @@
         public int? MaxGrowBy { get; set; }
 
         /// <summary>
-        /// Creates an instance of an <see cref="ArrayBufferWriter{T}"/>, in which data can be written to,
+        /// Creates an instance of an <see cref="TestArrayBufferWriter{T}"/>, in which data can be written to,
         /// with an initial capacity specified.
         /// </summary>
         /// <param name="initialCapacity">The minimum capacity with which to initialize the underlying buffer.</param>
         /// <exception cref="ArgumentException">
         /// Thrown when <paramref name="initialCapacity"/> is not positive (i.e. less than or equal to 0).
         /// </exception>
-        public ArrayBufferWriter(int initialCapacity)
+        public TestArrayBufferWriter(int initialCapacity)
         {
             if (initialCapacity <= 0)
                 throw new ArgumentException(nameof(initialCapacity));
@@ -111,7 +111,7 @@
         /// Clears the data written to the underlying buffer.
         /// </summary>
         /// <remarks>
-        /// You must clear the <see cref="ArrayBufferWriter{T}"/> before trying to re-use it.
+        /// You must clear the <see cref="TestArrayBufferWriter{T}"/> before trying to re-use it.
         /// </remarks>
         public void Clear()
         {