[bind] Provide a way to transform values during the bind

[jkb0o]

For now you can bind only values of the same type:

bind!(player, Player.name => name, Label.value)

The Player.name and Label.value are string types.
It should be possile to provide some transformer to the bind:

bind!(player, Health.value => |v| format!("Health:  {}", v) => label, Label.value)

While the general form should cover all cases, I believe the format! is the most often scenario, so bind! macro should treat string literals as format. The previous example will looks like this:

bind!(player, Health.value => "Health:  {}" => label, Label.value)

From my point of view, transformers should be a part of BindTo only, but looks like it is much simpler to make them a part of BindFrom, at least as the first step.