add trim_left and trim_right, add setup static asserts, update type_traits, refactor some code, add unit tests

include/ss/common.hpp

@@ -0,0 +1,19 @@
+#pragma once
+#include <vector>
+
+namespace ss {
+
+struct none {};
+
+using string_range = std::pair<const char*, const char*>;
+using split_data = std::vector<string_range>;
+
+constexpr inline auto default_delimiter = ",";
+
+template <bool StringError>
+inline void assert_string_error_defined() {
+    static_assert(StringError,
+                  "'string_error' needs to be enabled to use 'error_msg'");
+}
+
+} /* ss */