Create a more clear boundary between "library" and "client"

Even though "abcs" is technically a front-end for
libab, what it does is fill in some blanks left available by
it. However, I'm keeping most of this "fill-in-the-blanks"
code standalone, so that it may be used in other projects.

include/abacus.hpp

@@ -24,6 +24,7 @@ class abacus {
         void add_operator_infix(const std::string& op, const std::string& func, int assoc, int prec);
         void add_operator_prefix(const std::string& op, const std::string& func);
         void add_operator_postfix(const std::string& op, const std::string& func);
+        void add_standard();
         ref run(const std::string& code);
         template <typename ... Ts>
         ref call(const std::string& bane, Ts...params);