Disallow function definition and non-primitive function applications with names that start with an underscore

This MR makes the compiler raise a gentle error if it finds a function definition or application that starts with an _ during parsing.

In a recent meeting we (Bodo, Jordy, David and me) discovered that it is in fact possible for the programmer to make a function that starts with an underscore.

This should not be the case because the idea is that only the compiler can generate functions that start with an _ this way there is never a conflict between compiler generated functions and functions from the programmer.

I made sure that you can still call primitive functions.