Add in modifier to projection function so that readonly structs won't be copied when applying the projection.

Author: MendelMonteiro

src/StructLinq.Benchmark/ArrayClassSum.cs

@@ -50,9 +50,10 @@ public Container(int element)
                 Element = element;
             }
         }
+
         private struct Select : IFunction<Container, int>
         {
-            public int Eval(Container element)
+            public int Eval(in Container element)
             {
                 return element.Element;
             }

src/StructLinq.Benchmark/RangeWhereSelectSum.cs

@@ -62,15 +62,15 @@ public int SysSum()
 
     struct WherePredicate : IFunction<int, bool>
     {
-        public bool Eval(int element)
+        public bool Eval(in int element)
         {
             return (element & 1) == 0;
         }
     }
 
     struct SelectFunction : IFunction<int, int>
     {
-        public int Eval(int element)
+        public int Eval(in int element)
         {
             return element * 2;
         }

src/StructLinq.Benchmark/Select.cs

@@ -65,7 +65,7 @@ public int ConvertSelect()
 
     struct MultFunction : IFunction<int, double>
     {
-        public double Eval(int element)
+        public double Eval(in int element)
         {
             return element * 2.0;
         }

src/StructLinq.Benchmark/Where.cs

@@ -68,7 +68,7 @@ public int ConvertSelect()
 
     struct WhereFunc : IFunction<int, bool>
     {
-        public bool Eval(int element)
+        public bool Eval(in int element)
         {
             return element > 0;
         }

src/StructLinq.Tests/SelectTests.cs

@@ -46,7 +46,7 @@ protected override IEnumerable<int> Build(int size)
 
         struct MultFunction : IFunction<int, double>
         {
-            public double Eval(int element)
+            public double Eval(in int element)
             {
                 return element * 2.0;
             }

src/StructLinq.Tests/WhereTests.cs

@@ -46,7 +46,7 @@ public void StructTest()
 
         struct WhereFunc : IFunction<int, bool>
         {
-            public bool Eval(int element)
+            public bool Eval(in int element)
             {
                 return element > 0;
             }

src/StructLinq/Functions.cs

@@ -19,7 +19,7 @@ public StructFunction(Func<TIn, TOut> inner)
         {
             this.inner = inner;
         }
-        public TOut Eval(TIn element)
+        public TOut Eval(in TIn element)
         {
             return inner(element);
         }

src/StructLinq/IFunction.cs

@@ -1,7 +1,7 @@
 namespace StructLinq
 {
-    public interface IFunction<in TIn, out TOut>
+    public interface IFunction<TIn, out TOut>
     {
-        TOut Eval(TIn element);
+        TOut Eval(in TIn element);
     }
 }