Merge or-suggestions.hh into suggestions.hh

No real need for keeping a separate header for such a simple class.

This requires changing a bit `OrSuggestions<T>::operator*` to not throw
an `Error` to prevent a cyclic dependency. But since this error is only
thrown on programmer error, we can replace the whole method by a direct
call to `std::get` which will raise its own assertion if needs be.

src/libexpr/eval-cache.hh

@@ -3,7 +3,6 @@
 #include "sync.hh"
 #include "hash.hh"
 #include "eval.hh"
-#include "or-suggestions.hh"
 
 #include <functional>
 #include <variant>