Clarify class/struct preference

Classes and structs both have their place, and it is confusing to always
prefer one of them. It depends on the situation.
The reworded version follows C++ Core Guideline C.2:
"Use class if the class has an invariant; use struct if the data members
can vary independently"

CODING_STANDARD.md

@@ -185,7 +185,8 @@ Formatting is enforced using clang-format. For more information about this, see
 # C++ features
 - Do not use namespaces, except for anonymous namespaces.
 - Prefer use of `using` instead of `typedef`.
-- Prefer use of `class` instead of `struct`.
+- Prefer use of `class` instead of `struct` if there is an invariant between
+  members.
 - Write type modifiers before the type specifier.
 - Make references `const` whenever possible
 - Make member functions `const` whenever possible