Developer-visible task state readout

It would be nice to have a way for tasks to append some sort information to the output of humility tasks. For instance, to indicate that the network link managed by a task is up. This could remove a lot of squinting at ringbuffers/readvar.

My sketch of a design:

• Extend the debugger-to-task DBI with a new symbol, something like HUMILITY_TASK_STATUS.
• Tasks can declare this symbol at top level (static no_mangle).
• Any value stored in such a global variable will be read out by Humility and printed using the standard DWARF pretty-printing mechanism.

So, to continue the network stack example, one could do this:

struct NetStatus {
    link: LinkState,
    mac: [u8; 6],
}

enum LinkState { Up, Down, Confused }

#[no_mangle]
static HUMILITY_TASK_STATUS: StaticCell<NetStatus> = StaticCell::new(NetStatus {
    link: LinkState::Down,
    mac: [0xFF; 6],
});

...and then subsequently update it:

HUMILITY_TASK_STATUS.borrow_mut().mac = generated_mac;

and then over in Humility we'd extend the task status output, maybe only on request, to be something like

11 net                  339   2 wait: send to gimlet_seq/gen19   NetStatus { link: Up, mac: [1, 2, 3, 4, 5, 6] }

@mkeeter and I were talking about this recently, and I believe I've previously run it by @bcantrill.

Start by tracing the debugger-to-task DBI and the existing Humility task status output. Determine how a HUMILITY_TASK_STATUS symbol could be declared by Rust tasks and read through DWARF pretty-printing. Done means task status output can include the declared value, such as the NetStatus example, without requiring ringbuffers or readvar.