characters in quotes aren't escaped correctly

It looks like some characters in quotes aren't escaped correctly.

I haven't extensively tested this, but here is one example I've found:

```purescript
module Main where

example :: String
example = "\""
```

This compiles to:

```nix
let
  module = { };
  example = """;
in
  {inherit example;}
```

But `example` should probably be `"\""`.