Fix some code smells

[oleg-derevenetz]

• C++20 introduced the addressing restriction for stdlib functions (perhaps in order to "bless" the potential presence of "stdlib functions" that are not in fact functions - e.g. macros), so it's better be safe than sorry, especially if it doesn't cost anything;
• Rename various xxx_t identifiers;
• Move some module-local types & code to the anonymous namespace;
• Make the skill_static.h - related code more clear.