Give deferred function guards the ability to run before destruction

Created by: bcaimano

This commit adds three new functions to kj::_:Deferred:

• A move-assignment operator to allow reuse of the same variable for deferred functors of the same type.
• A run() function to immediately run the guard before destruction.
• A nullptr assignment operator (nullification) as syntactic sugar for run().

It's worth mentioning that via the magic of kj::Maybe and universal references, Deferred maintains the ability to hold lvalue reference to functors instead of functor instances.