Custom constraints can't be implemented without access to `make_cgreen_<type>_value()`

[thoni56]

The feature of being able to create custom constraints (see https://cgreen-devs.github.io/#_custom_constraints) is very handy.

However the example, and use, requires the inclusion of cgreen_value_internal.h which is not available in a packaged installation, so you need to have the source available.

One way to fix this is to move cgreen_value_internal.h to include/cgreen/internal/cgreen_value_internal.h. This way it would be available in packaged installations, but also clearly signal that using this is subject to some risk.

[thoni56]

The example also shouldn't` indicate that you should (or even can) access Cgreen's internal functions...