* Added syntactic sugar to the construction of attribute sets to

`inherit' variables from the surrounding lexical scope.

  E.g.,

    {stdenv, libfoo}: derivation {
      builder = ./bla;
      inherit stdenv libfoo;
      xyzzy = 1;
    }

  is equivalent to

    {stdenv, libfoo}: derivation {
      builder = ./bla;
      stdenv = stdenv;
      libfoo = libfoo;
      xyzzy = 1;
    }

  Note that for mutually recursive attribute set definitions (`rec
  {...}'), this also works, that is, `rec {inherit x;}' is equivalent
  to `let {fresh = x; body = rec {x = fresh;};}', *not*
  `rec {x = x}'.

src/libexpr/lexer.l

@@ -50,6 +50,7 @@ else        { return ELSE; }
 assert      { return ASSERT; }
 let         { return LET; }
 rec         { return REC; }
+inherit     { return INHERIT; }
 
 \=\=        { return EQ; }
 \!\=        { return NEQ; }