Where the ULP "hal" code should reside

[ivmarkov]

(Not a top priority) the ULP "hal" (just a subset of the GPIO pins which have RTC capabilities really) currently lives inside esp-idf-hal, yet it is not using the esp-idf-sys crate or anything from ESP-IDF. It might be better off in its own crate, or even inside the esp32-hal crate (or a forest of crates in future), as this way it might take advantage of the PAC crates and the SVD work. And it is really "bare metal" after all.