Added WaitForEdgeForever and WaitForStateForever transactions, which return a Pending future to the callee.

[leighleighleigh]

I have some code I'd like to mock for a quadrature encoder, which uses select(pin_clk,pin_data).
In order to properly test it, I need to be able to mock different paths in the select statement.

As it stands currently, the current async transactions always return Ok() when called - this means my select() paths are always taking the first branch.

The solution I propose here is to add a modified WaitForEdge/WaitForState enum called WaitFor{State,Edge}Forever, which returns a Pending future. This simulates a pin that never changes, while still allowing a mock transaction for the actual wait_for_edge call.

I have added tests for this, which poll the returned future for 10 milliseconds before timing-out (successful result).

[ryankurte]

seems good to me!

[dbrgn]

Thanks for the review, @ryankurte!