[jnimarshalmethod-gen] Avoid creating AppDomains (#720)

This is part of getting `jnimarshalmethod-gen` to work on Windows and
later with .NET 5.

The created `AppDomain` was used to isolate processed assembly, so that
we could unload it and rewrite with generated changes; see 2a9ac6a.

That didn't work on Windows even with .NET Framework, which has
the full `AppDomain` API.  In .NET 5, `AppDomain`s can't be created.

Instead of using `AppDomain`s, pre-load the assembly in memory and
load it from there.  This avoids the file sharing issues.

Author: radekdoulik

tools/jnimarshalmethod-gen/App.cs

@@ -39,17 +39,13 @@ class App : MarshalByRefObject
 
 		public static int Main (string [] args)
 		{
-			var domain = AppDomain.CreateDomain ("workspace");
-			var app = (App)domain.CreateInstanceAndUnwrap (typeof (App).Assembly.FullName, typeof (App).FullName);
-
+			var app = new App ();
 			app.AddMonoPathToResolverSearchDirectories ();
 
 			var assemblies = app.ProcessArguments (args);
 			app.ProcessAssemblies (assemblies);
 			var filesToDelete = app.FilesToDelete;
 
-			AppDomain.Unload (domain);
-
 			foreach (var path in filesToDelete)
 				File.Delete (path);
 
@@ -282,8 +278,7 @@ public int Compare (MethodInfo a, MethodInfo b)
 
 		void CreateMarshalMethodAssembly (string path)
 		{
-			var assembly        = Assembly.LoadFile (Path.GetFullPath (path));
-
+			var assembly        = Assembly.Load (File.ReadAllBytes (Path.GetFullPath (path)));
 			var baseName        = Path.GetFileNameWithoutExtension (path);
 			var assemblyName    = new AssemblyName (baseName + "-JniMarshalMethods");
 			var fileName        = assemblyName.Name + ".dll";