Fix printer.process macro to accept multiple values

[hageboeck]

This is #566, but rebased after fixing the CI.

Original PR description:

This Pull request fixes the following issue:
When implementing the operators, some would want to write CHECK(p1,three == Fraction{3,1});, but the compiler complains with the following error:

operators.cpp:109:34: error: macro ‘CHECK’ passed 3 arguments, but takes just 2
  109 |   CHECK(p1,three == Fraction{3,1}); //why double parenthesis
      |                                  ^
operators.cpp:93:9: note: macro ‘CHECK’ defined here
   93 | #define CHECK(printer, what) printer(#what, what)
      |         ^~~~~
operators.cpp: In function ‘int main()’:
operators.cpp:109:3: error: ‘CHECK’ was not declared in this scope
  109 |   CHECK(p1,three == Fraction{3,1}); //why double parenthesis
      |   ^~~~~
make: *** [Makefile:8: operators] Error 1

So it is mandatory to include an additional (), like so: CHECK(p1,(three == Fraction{3,1}));

This Pull request makes it possible to write it in a shorter form: CHECK(p1,three == Fraction{3,1});