Add reset signal to `FlipFlop`

[mkorbel1]

Motivation

It would be nice to use FlipFlop with a reset signal (optionally synchronous or asynchronous)

Desired solution

Add optional named parameter(s) to FlipFlop that, when specified, add reset behavior.

Alternatives considered

People can already achieve this using the Sequential block, but FlipFlop is convenient.

[stephtancs]

i can do this

[mkorbel1]

While we're at it, we should really make d and clk private in FlipFlop

[chykon]

But then the use of FlipFlop will become not so convenient: you will have to use input(). Or am I not understanding something?

UPDATE: Yes, I misunderstood this. The clk and d inputs can be hidden since we connect to them in the FlipFlop constructor.