This introduces a `Pender` struct with enum cases for thread-mode, interrupt-mode and
custom callback executors. This avoids calls through function pointers when using only
the thread or interrupt executors. Faster, and friendlier to `cargo-call-stack`.
`embassy-executor` now has `arch-xxx` Cargo features to select the arch and to enable
the builtin executors (thread and interrupt).
This brings it inline with the other embassy-usb descriptor APIs and allows it to integrate well with the Builder to allow class constructors to add MS OS descriptors.
Also adds a `usb_serial_winusb` example to demonstrate how to use the API.
