More code clean-up

***NO_CI***

Author: josesimoes

nanoFramework.Runtime.Native/ExecutionConstraint.cs

@@ -1,6 +1,9 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System;
+using System.Runtime.CompilerServices;
+
 namespace nanoFramework.Runtime.Native
 {
     /// <summary>

nanoFramework.Runtime.Native/GC.cs

@@ -1,6 +1,8 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Runtime.CompilerServices;
+
 namespace nanoFramework.Runtime.Native
 {
     /// <summary>

nanoFramework.Runtime.Native/Power.cs

@@ -1,6 +1,8 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Runtime.CompilerServices;
+
 namespace nanoFramework.Runtime.Native
 {
     /// <summary>

nanoFramework.Runtime.Native/Properties/AssemblyInfo.cs

@@ -1,12 +1,12 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
-using System.Reflection;
-using System.Runtime.InteropServices;
-
 // General Information about an assembly is controlled through the following 
 // set of attributes. Change these attribute values to modify the information
 // associated with an assembly.
+using System.Reflection;
+using System.Runtime.InteropServices;
+
 [assembly: AssemblyTitle("nanoFramework.Runtime.Native")]
 [assembly: AssemblyCompany("nanoFramework Contributors")]
 [assembly: AssemblyProduct("nanoFramework.Runtime.Native")]

nanoFramework.Runtime.Native/RTC.cs

@@ -1,6 +1,8 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Runtime.CompilerServices;
+
 namespace nanoFramework.Runtime.Native
 {
     /// <summary>

nanoFramework.Runtime.Native/System/Environment.cs

@@ -1,6 +1,8 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Runtime.CompilerServices;
+
 namespace System
 {
     /// <summary>

nanoFramework.Runtime.Native/SystemInfo.cs

@@ -1,6 +1,9 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System;
+using System.Runtime.CompilerServices;
+
 namespace nanoFramework.Runtime.Native
 {
     /// <summary>