Add some sort of iterator for repeated registers

[diondokter]

Currently you can do:

for i in 0..10 {
    device.foo(i).write(|_|);
}

This is annoying. There's no way to get the max even.

Instead, something like this should be made possible:

for foo in  device.iter_foo() {
  foo .write(|_|);
}

However, this would introduce a new name that can clash, so don't know how to solve that: related #183

[diondokter]

Idea!

What if foo doesn't take an index, but returns an object that implements Index?

It'd look like this:

// Single element
device.foo()[0].write(|_|);
// Range
for foo in device.foo()[2..6] {}
// Just get all of them
for foo in device.foo()[..] {}

All of this, without needing to introduce a new name.

Edit:
The index trait can't work since it returns a reference and we need to return an actual instance. But we can still make foo take a generic index parameter that determines which type is returned