Class mapping

include/covscript/impl/symbols.hpp:158–174

	template <typename Key, typename T>
	class mapping final {
		std::map<Key, T> mDat;

	public:
		mapping(std::initializer_list<std::pair<const Key, T>> l) : mDat(l) {}

		bool exist(const Key &k) const {
			return mDat.count(k) > 0;
		}

		const T &match(const Key &k) const
		{
			if (!exist(k))
				throw compile_error("Undefined Mapping.");
			return mDat.at(k);
		}
	};

	class token_base {
		friend class compiler_type;