Add visibility protection for `$TestsOnly$` in C++ extensions.

[ta0kira]

For example, guard the headers with a macro (e.g., ZEOLITE_TESTS_ONLY) so that the category is only available if that macro is defined.

It would also be helpful if there was a consequence of defining that macro so that it can't just be used arbitrarily.

[ta0kira]

In order to prevent a non-$TestsOnly$ category from being defined in a $TestsOnly$ C++ extension, we can probably use some macro trickery:

// In non-$TestsOnly$ Streamlined_Foo.hpp.

#ifdef ZEOLITE_TESTS_ONLY
#error Category Foo cannot be defined as $TestsOnly$
#endif

#ifndef ZEOLITE_NO_TESTS_ONLY
#define ZEOLITE_NO_TESTS_ONLY
#endif

// In $TestsOnly$ Category_Bar.hpp.

#if !defined(ZEOLITE_TESTS_ONLY) || defined(ZEOLITE_NO_TESTS_ONLY)
#error Category Bar can only be used in $TestsOnly$ sources
#endif 

The key here is that both the streamlined header (#included to define the category) and category header (#included to use the category) are generated.

• // Attempt to circumvent.
  
  #include "Streamlined_Foo.hpp"
  #define ZEOLITE_TESTS_ONLY
  #include "Category_Bar.hpp"  // <- #error here due to ZEOLITE_NO_TESTS_ONLY

• // Attempt to circumvent.
  
  #define ZEOLITE_TESTS_ONLY
  #include "Category_Bar.hpp"
  #include "Streamlined_Foo.hpp"  // <- #error here due to ZEOLITE_TESTS_ONLY

• // Attempt to circumvent.
  
  #include "Streamlined_Foo.hpp"
  #define ZEOLITE_TESTS_ONLY
  #undef ZEOLITE_NO_TESTS_ONLY
  #include "Category_Bar.hpp"  // Fine, this would actually work.

These two macros should be avoidable in all hand-written code (once streamlined extensions are implemented); therefore, they could be given ugly names that would look bad in hand-written code, e.g., ZEOLITE_TESTS_ONLY_GUARD_FOR_USE_IN_CODEGEN. Or, the macro names could include the compiler hash at the end of the name, e.g., ZEOLITE_TESTS_ONLY_7bc4535736d2796f.

[ta0kira]

A more concise plan:

• In any $TestsOnly$ streamlined header (e.g., Streamlined_Foo.hpp):

  #ifndef ZEOLITE_TESTS_ONLY_7bc4535736d2796f
  #define ZEOLITE_TESTS_ONLY_7bc4535736d2796f
  #endif
  
  #ifdef ZEOLITE_NO_TESTS_ONLY_7bc4535736d2796f
  #error Cannot define both $TestsOnly$ and non-$TestsOnly$ in the same source file
  #endif

• In any non-$TestsOnly$ streamlined header (e.g., Streamlined_Foo.hpp):

  #ifndef ZEOLITE_NO_TESTS_ONLY_7bc4535736d2796f
  #define ZEOLITE_NO_TESTS_ONLY_7bc4535736d2796f
  #endif
  
  #ifdef ZEOLITE_TESTS_ONLY_7bc4535736d2796f
  #error Cannot define both $TestsOnly$ and non-$TestsOnly$ in the same source file
  #endif

• In any $TestsOnly$ category header (e.g., Category_Bar.hpp):

  #ifndef ZEOLITE_TESTS_ONLY_7bc4535736d2796f
  #error Category Bar can only be used by $TestsOnly$ categories
  #endif

The key points are that:

• Streamlined headers are only #included by source files that will define the respective categories, so disallowing non-$TestsOnly$ in $TestsOnly$ sources is fine.
• Category headers are only #included where the category will be used, notwithstanding other generated files, so non-$TestsOnly$ can be used in $TestsOnly$ sources.