Add check for external procedures

[ZedThree]

Closes #23

There's a sort of naming clash with external-function -- I've called
this external-statement, but I think it's maybe better as
external-procedure. I wonder if that might be better as something
like procedure-not-in-module?

[LiamPattinson]

👍

[LiamPattinson]

There's a sort of naming clash with external-function -- I've called
this external-statement, but I think it's maybe better as
external-procedure. I wonder if that might be better as something
like procedure-not-in-module?

I think procedure-not-in-module and external-procedure would be the best names, but it'll be hard to avoid some confusion since they're both arguing against the same practice but from different angles. We could set up some redirects for this, but I feel like having both external-function and external-procedure pointing at different rules might be too confusing, so I think a breaking change may be in order. Maybe raise an issue for this?