Improve handling ability for result type.

[kekyo]

Will validate return type both Task and ValueTask.

public Task<int> Sum(int a, int b) => Task.FromResult(a + b);
public ValueTask<int> Sum(int a, int b) => new ValueTask<int>(a + b);

Will validate any constant return type with synchronized method/function.

.NET:

public int Sum(int a, int b) => a + b;

// Requires dirty naming (ex: namespace).
using DupeNukem.Synchronized;
messenger.RegisterFunc<int, int, int>(this.Sum);

JavaScript:

/* async */ function add(a, b) {
  return a + b;
}