SPI and related changes (#2681)

* Define Octal SpiDataMode

* Fix BitOrder capitalization

* Hide DmaError

* Derive more traits

* Return error for unsupported half-duplex parameter combinations

* Remove unnecessary implementation detail

* Return error instead of silently breaking transaction

* Downgrade assert to debug_assert

* Expand on the enum variants

* Remove redundant imports

* Hide some unstable functions

esp-hal/src/peripheral.rs

@@ -18,6 +18,8 @@ use core::{
 ///
 /// but it is the size of `T` not the size
 /// of a pointer. This is useful if T is a zero sized type.
+#[derive(Debug, PartialEq, Eq)]
+#[cfg_attr(feature = "defmt", derive(defmt::Format))]
 pub struct PeripheralRef<'a, T> {
     inner: T,
     _lifetime: PhantomData<&'a mut T>,