Returns escape string for char or nil if none
;; simple examples
user=> (char-escape-string \
ewline)
"\\\
"
user=> (char-escape-string \\c) ; no escape sequence for 'c'
nil
user=> (char-escape-string \\tab)
"\\\\t"
user=> (char-escape-string \\backspace)
"\\\\b"
user=>