Make operations on pins take mutable reference

[Kixunil]

I believe it'd be more logical to take mutable reference to pins. I know that memory itself doesn't change, however, there are good reasons to require mutability anyway:

• Accessing the pin from multiple threads probably isn't what people want to do by accident. (They still can do it on purpose.)
• Requiring mutable reference allows people to write mocks and simulate the pins in tests.
• It's more natural to use mutable references when you change something.
• It clearly shows that some state is being mutated.

What do you think?

[SaberYoun6]

How would your try and impliment this into the code?