Add support for local functions

[maca88]

Example:

public class Simple
{
  public void Test()
  {
    void Read()
    {
      SimpleFile.Read();
    }
    Read();
  }
}

should be converted to:

public partial class Simple
{
  public Task TestAsync()
  {
    Task ReadAsync()
    {
      return SimpleFile.ReadAsync();
    }
    return ReadAsync();
  }
}