ascii: add try_lower

ascii.hpp

@@ -131,6 +131,19 @@ namespace cruft::ascii {
     }
 
 
+    ///------------------------------------------------------------------------
+    /// Convert an ASCII character into a lower case character if it is upper
+    /// case, else return it unchanged.
+    ///
+    /// If you know the character is always upper case then prefer to_lower.
+    constexpr inline
+    char
+    try_lower (char c) noexcept
+    {
+        return is_upper (c) ? to_lower (c) : c;
+    }
+
+
     ///////////////////////////////////////////////////////////////////////////
     /// Returns true if the supplied ASCII character is whitespace.
     constexpr inline