ascii: add try_upper transform

ascii.hpp

@@ -117,6 +117,19 @@ namespace cruft::ascii {
     }
 
 
+    ///------------------------------------------------------------------------
+    /// Convert an ASCII character into a upper case character if it is lower
+    /// case, else return it unchanged.
+    ///
+    /// If you know the character is always lower case then prefer to_upper.
+    constexpr inline
+    char
+    try_upper (char c) noexcept
+    {
+        return is_lower (c) ? to_upper (c) : c;
+    }
+
+
     ///------------------------------------------------------------------------
     /// Converts an upper case ASCII character into a lower case character.
     ///