Add a Dbgmcu struct capable of enabling all relevant DBGMCU.cr fields.

Includes the addition of a `dbgmcu!(...)` macro table which currently takes
the form of

	(cr, $fn_name:ident)

where `$fn_name` is something like `set_dbgsleep_d1` etc.

The method is unsafe, since it's performing unsafe PAC operations.

Two examples modified to demonstrate its usage.

embassy-stm32/src/lib.rs

@@ -31,6 +31,8 @@ pub mod adc;
 pub mod clock;
 #[cfg(dac)]
 pub mod dac;
+#[cfg(dbgmcu)]
+pub mod dbgmcu;
 #[cfg(all(eth, feature = "net"))]
 pub mod eth;
 #[cfg(exti)]