Line numbers are messed up with Execution using trampoline

[johnynek]

probably need to add algebird here:

https://github.com/twitter/scalding/blob/develop/scalding-core/src/main/scala/com/twitter/scalding/LineNumber.scala#L62

We should also first make a failing unit test that gets the line numbers wrong using executions to avoid regression.

/cc @jnievelt @ianoc