Incorrect handling of single-quoted escape sequences in `ast.ml`

See `test/failing/backslash` or `test/failing/aaaa_single` (which comes from @tucak's original bug report's reproducer (mgree/smoosh#5) for the bug resolved in #1).

The problem is, depending on your perspective, in backslash insertion or in `E` insertion in `ocaml/ast.ml`. At this point, `ast.ml` is pretty out of sync with `mgree/smoosh`'s `shim.ml`. Hopefully upcoming student work on generalizing the parsing interface will give us a simpler model, though we're probably a ways out.