Change closures to use $app as the first parameter.

Almost always the first parameter is $app therefor it makes little sense
to use `use` part of the closure and having PHP do context switching all
over the place.

This also makes it explicit in the service providers exactly what $app
instance is used.
Agree with @stof that we should not do it in this case. The initializer takes no args. Also keep BC in mind please.