* New language feature: with expressions.

The expression `with E1; E2' evaluates to E2 with all bindings in
  the attribute set E1 substituted.  E.g.,

    with {x = 123;}; x

  evaluates to 123.  That is, the attribute set E1 is in scope in E2.

  This is particularly useful when importing files containing lots
  definitions.  E.g., instead of

    let {
      inherit (import ./foo.nix) a b c d e f;

      body = ... a ... f ...;
    }

  we can now say

    with import ./foo.nix;

    ... a ... f ...

  I.e., we don't have to say what variables should be brought into scope.

src/libexpr/lexer.l

@@ -48,6 +48,7 @@ if          { return IF; }
 then        { return THEN; }
 else        { return ELSE; }
 assert      { return ASSERT; }
+with        { return WITH; }
 let         { return LET; }
 rec         { return REC; }
 inherit     { return INHERIT; }