counters: Array-of-Counters

[jamesmunns]

oxidecomputer/hubris#2668 added a manual counters impl that was of the shape:

[AtomicU32; N]

rather than

struct {
    ...: AtomicU32,
    // ...
}

I intended to get this merged before landing 2668, but failed to do so. This should rectify that.

This will potentially help support oxidecomputer/hubris#2675 in the future, when we can automatically derive this rather than manually implement.