Add free function to UsbAllocator

[jnthbdn]

Allowing users to free the UsbBus from the UsbAllocator.

[eldruin]

I am not sure it makes sense to allow this.
Maybe somebody else can comment like @ryan-summers ?

[ryan-summers]

Yeah I'm also not sure this makes sense to allow. Can you elaborate on what the necessary use case is here?

[jnthbdn]

Hello,

Thank you for your feedback. I've added this function to make the link with the PR725 (rp-rs/rp-hal#725) from rp-hal.
The problem is that creating a UsbBus requires ownership of two peripherals (USBCTRL_REGS and USBCTRL_DPRAM) and a clock. But if you want to stop the USB, it's not possible to recover these resources, and you end up, for example, with a Partially move error (in my case from the clockManager).

[ianrrees]

It would be nice to have the ability to tear down structures - I could imagine this being useful for implementing a low power device that usually keeps the USB peripheral powered down, for instance.

However, I see two issues with this implementation:

1. Any allocated endpoints will essentially be left dangling. So, I think there would need to be destructors added to the endpoints, and some bookkeeping so that the free methods behave correctly.
2. UsbDevice::build() calls UsbBusAllocator::freeze(), so it seems like the new free() wouldn't work as desired after build().