* Added a new kind of multi-line string literal delimited by two

single quotes.  Example (from NixOS):

    job = ''
      start on network-interfaces

      start script

        rm -f /var/run/opengl-driver
        ${if videoDriver == "nvidia"        
          then "ln -sf ${nvidiaDrivers} /var/run/opengl-driver"
          else if cfg.driSupport
          then "ln -sf ${mesa} /var/run/opengl-driver"
          else ""
        }

        rm -f /var/log/slim.log

      end script
    '';

  This style has two big advantages:

  - \, ' and " aren't special, only '' and ${.  So you get a lot less
    escaping in shell scripts / configuration files in Nixpkgs/NixOS.
    The delimiter '' is rare in scripts (and can usually be written as
    "").  ${ is also fairly rare.

    Other delimiters such as <<...>>, {{...}} and <|...|> were also
    considered but this one appears to have the fewest drawbacks
    (thanks Martin).

  - Indentation is intelligently stripped so that multi-line strings
    can follow the nesting structure of the containing Nix
    expression.  E.g. in the example above 6 spaces are stripped from
    the start of each line.  This prevents unnecessary indentation in
    generated files (which sometimes even breaks things).

  See tests/lang/eval-okay-ind-string.nix for some examples.

src/libexpr/nixexpr-ast.def

@@ -46,6 +46,9 @@ Int | int | Expr |
 Str | string ATermList | Expr |
 Str | string | Expr | ObsoleteStr
 
+# Internal to the parser, doesn't occur in ASTs.
+IndStr | string | Expr |
+
 # A path is a reference to a file system object that is to be copied
 # to the Nix store when used as a derivation attribute.  When it is
 # concatenated to a string (i.e., `str + path'), it is also copied and