`a`, `c`, and `i` without `\` should be rejected

[e-kwsm]

fee1ff3

$ gnused -e a <<< x && echo $?
gnused: -e expression #1, char 1: expected \ after `a', `c' or `i'
$ bsdsed -e i <<< x && echo $?
bsdsed: 1: "i
": command i expects \ followed by text
$ ./target/debug/sed -e c <<< x && echo $?

0