Adds the `rt` feature which disables `esp_hal::init` and removes
runtime symbols from the global name space.
Disabling the `rt` feature is the recommended way to use esp-hal as a
library.
* Use #[instability::unstable] where possible, cleanup and improve consistency
* De-duplicate marking SpiDma as unstable
* Remove #[unstable] from private structs, functions and inherent impl blocks, address review comments
* Don't mark uWrite as unstable
* API guidelines entry
* mark uWrite unstable again
* Make SetConfig and InterruptConfigurable consistent and add inherent set_interrupt_handler functions vol.2
* warnings
