[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
 
 publicstaticintMain(string[]args)
 {
-vardomain=AppDomain.CreateDomain("workspace");
-varapp=(App)domain.CreateInstanceAndUnwrap(typeof(App).Assembly.FullName,typeof(App).FullName);
-
+varapp=newApp();
 app.AddMonoPathToResolverSearchDirectories();
 
 varassemblies=app.ProcessArguments(args);
 app.ProcessAssemblies(assemblies);
 varfilesToDelete=app.FilesToDelete;
 
-AppDomain.Unload(domain);
-
 foreach(varpathinfilesToDelete)
 File.Delete(path);
 
@@ -282,8 +278,7 @@ public int Compare (MethodInfo a, MethodInfo b)
 
 voidCreateMarshalMethodAssembly(stringpath)
 {
-varassembly=Assembly.LoadFile(Path.GetFullPath(path));
-
+varassembly=Assembly.Load(File.ReadAllBytes(Path.GetFullPath(path)));
 varbaseName=Path.GetFileNameWithoutExtension(path);
 varassemblyName=newAssemblyName(baseName+"-JniMarshalMethods");
 varfileName=assemblyName.Name+".dll";