COMMON: Add underlying type names to Common::String.

The value_type is analogous to std::basic_string::value_type.
The unsigned_type on the other hand is an unsigned type of the value_type which
allows to obtain an character without nasty sign extension.

common/str.h

@@ -234,6 +234,8 @@ public:
 	static String vformat(const char *fmt, va_list args);
 
 public:
+	typedef char          value_type;
+	typedef unsigned char unsigned_type;
 	typedef char *        iterator;
 	typedef const char *  const_iterator;