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

[mgree]

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.