+Convert a character index (e.g., the \eN\(aq\(aq escape) into a
+printable ASCII character.
+Returns \e0 (the nil character) if the input sequence is malformed.
+.It Fn mchars_num2uc
+Convert a hexadecimal character index (e.g., the \e[uNNNN] escape) into
+a Unicode codepoint.