Add option to change name when using @:forward

[gene-pavlovsky]

It would be nice if it was possible to specify a custom name when using @:forward.
Sometimes, a name might conflict with an existing one. Sometimes, it can be for reason of readability.
I'm also thinking sometimes it would be useful to be able to declare a forwarded field as private.
One option for syntax

@:forward(
  length,
  getItem = get,
  private setItem = set
) var c:Container;