Get rid of the signal expression rewriter

This replaces the entire rewriter with more or less:

expressionToEval = "(function(<named params here>) { " + expr + " } )"

This also fixes crashes at run-time when the signal rewriter was
executed from the loader thread and tried to use a v4 identifier
hash with the same engine that's also in the main thread.

Change-Id: Ib1e4927d330706a593411fbff64ed3da1e23d0e0
Reviewed-by: Lars Knoll <lars.knoll@digia.com>