Disallow statements between overloads

Ensure that overloads are consecutive when checking functions.
this fixes #75 and #92 
This error message was changed from "Constructor implementation expected." to "Constructor implementation is missing.". Given that error is usually reported on the line that contains the last overload wording "Constructor implementation expected." is not clear enough to explain what went wrong (should I create implementation instead of this overloads? But what if I need this overloads? etc...). In opposite "Constructor implementation is missing." implies that this overload itself is ok but it lacks implementation and thus error is reported