Convert a char token, where the escape sequences (backslashes)
remain to be interpreted; raise Failure if an
incorrect backslash sequence is found; Token.Eval.char (Char.escaped c)
returns c
Taken.Eval.string strict s
Convert a string token, where the escape sequences (backslashes)
remain to be interpreted; raise Failure if strict and an
incorrect backslash sequence is found;
Token.Eval.string strict (String.escaped s) returns s