* Added plain lambdas, e.g., `let { id = x: x; const = x: y: x; }'.

`bla:' is now no longer parsed as a URL.

* Re-enabled support for the `args' attribute in derivations to
  specify command line arguments to the builder, e.g.,

    ...
    builder = /usr/bin/python;
    args = ["-c" ./builder.py];
    ...

src/libexpr/parser.y

@@ -54,6 +54,8 @@ expr: expr_function;
 expr_function
   : '{' formals '}' ':' expr_function
     { $$ = ATmake("Function(<term>, <term>)", $2, $5); }
+  | ID ':' expr_function
+    { $$ = ATmake("Function1(<term>, <term>)", $1, $3); }
   | expr_assert
   ;