Validation of invalid assigment (Type-Incompatibility - e.g. assign STRING to DINT)

[mhasel]

The following code snippet does not result in a validation error, even though it should:

PROGRAM FOO
  VAR
    x : DINT;
    s : STRING;
  END_VAR
    s := 'Hello, world!';
    x := s;
END_PROGRAM

We already have validations in place for invalid assignments, but as far as i can tell this validation only happens for either call parameters of when trying to assign characters. We should extend the validation to also cover other possibilities, otherwise the attempted codegen will result in a panic.

[99NIMI]

should be fixed with #780

[mhasel]

fixed with #780

[mhasel]

fixed with #780