Update swank to use file compiler entrypoint

[jbouwman]

The introduction of fuller package forms in coalton-lang/coalton#1091 also defines a file compiler entrypoint. Update swank to use this:

https://github.com/coalton-lang/coalton/blob/ab637bbf65ea10571bb40b6765111832e5af7224/src/entry.lisp#L262