[Count] add benchmark.

Author: reegeek

Documents/BenchmarksResults/ArrayWhereCount.md

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+## ArrayWhereCount
+
+### Source
+[ArrayWhereCount.cs](../../src/StructLinq.Benchmark/ArrayWhereCount.cs)
+
+### Results:
+``` ini
+
+BenchmarkDotNet=v0.12.1, OS=Windows 10.0.19042
+Intel Core i7-7700 CPU 3.60GHz (Kaby Lake), 1 CPU, 8 logical and 4 physical cores
+.NET Core SDK=5.0.102
+  [Host]     : .NET Core 5.0.2 (CoreCLR 5.0.220.61120, CoreFX 5.0.220.61120), X64 RyuJIT
+  DefaultJob : .NET Core 5.0.2 (CoreCLR 5.0.220.61120, CoreFX 5.0.220.61120), X64 RyuJIT
+
+
+```
+|                         Method |      Mean |     Error |    StdDev | Ratio | RatioSD | Gen 0 | Gen 1 | Gen 2 | Allocated |
+|------------------------------- |----------:|----------:|----------:|------:|--------:|------:|------:|------:|----------:|
+|                  HandmadedCode |  9.018 μs | 0.1786 μs | 0.3442 μs |  1.00 |    0.00 |     - |     - |     - |         - |
+|                        SysLinq | 28.411 μs | 0.1726 μs | 0.1614 μs |  3.13 |    0.12 |     - |     - |     - |      48 B |
+|         StructLinqWithDelegate | 31.017 μs | 0.6127 μs | 1.5259 μs |  3.47 |    0.24 |     - |     - |     - |      64 B |
+| StructLinqWithDelegateZeoAlloc | 30.736 μs | 0.6101 μs | 1.5195 μs |  3.42 |    0.25 |     - |     - |     - |         - |
+|         StructLinqWithFunction | 16.965 μs | 0.3245 μs | 0.4104 μs |  1.87 |    0.09 |     - |     - |     - |         - |

src/StructLinq.Benchmark/ArrayWhereCount.cs

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+using System.Linq;
+using BenchmarkDotNet.Attributes;
+
+namespace StructLinq.Benchmark
+{
+    [MemoryDiagnoser]
+    public class ArrayWhereCount
+    {
+        private const int Count = 10000;
+        private readonly int[] array;
+
+        public ArrayWhereCount()
+        {
+            array = Enumerable.Range(0, Count).ToArray();
+        }
+
+        [Benchmark(Baseline = true)]
+        public int HandmadedCode()
+        {
+            int count = 0;
+            for (int i = 0; i < array.Length; i++)
+            {
+                var elt = array[i];
+                if ((elt & 1) == 0)
+                {
+                    count++;
+                }
+            }
+            return count;
+        }
+        
+        [Benchmark]
+        public int SysLinq() => array
+                                .Where(x => (x & 1) == 0)
+                                .Count();
+
+        [Benchmark]
+        public int StructLinqWithDelegate() => array.ToStructEnumerable()
+                                                    .Where(x => (x & 1) == 0)
+                                                    .Count();
+
+        [Benchmark]
+        public int StructLinqWithDelegateZeoAlloc() => array.ToStructEnumerable()
+                                                            .Where(x => (x & 1) == 0, x=> x)
+                                                            .Count(x=>x);
+
+        [Benchmark]
+        public int StructLinqWithFunction()
+        {
+            var where = new WherePredicate();
+            return array
+                   .ToStructEnumerable()
+                   .Where(ref @where, x => x)
+                   .Count(x => x);
+        }
+    }
+}