add simple way to insert delays

Closes #184

I thought the proposed way of using an operator like >> to insert delays would be too easily confused with bit shifting, so I just created a normal function instead. I don't know if it's the best, but it works at least.

so now you can do the following to delay one of the inputs of the addition by 3T:

b = Constant(1)
c = Constant(1)
a = b + c
delayed_a = a.input(0).delay(3)