Implement `!` (std::ops::Not) for `Value`

[Lindenk]

Because of the way rust handles Enums vs Integers, there is no convenient way to invert a logic level with rust-wiringpi. I propose std::ops::Not be implemented for Value so that it can be inverted:

!High would be Low
!Low would be High

[Ogeon]

Good call! Will look at your PR. 🙂