Fortran: check for undeclared variables by forcing implicit none everywhere
C/C++: check for unused parameters and return types
removed unused variables from test cases
ci: do missing return and unused arg check with Github Actions
Compiler versions 15.0 and later actually ignore invalid values for the
-std= option unless `-diag-error 10159` is passed, so we need to put
that in the unit test.
I have tested this with versions 14.0.3, 15.0.6, 16.0.4, and 17.0.1.
Would be great if someone could test with 13.x.y
