Add repr(transparent) to Reg struct

To guarantee that the register has the proper layout so that the cast
of an integer to a *const RegisterBlock is sound.

Author: couchand

src/generate/generic.rs

@@ -34,6 +34,7 @@ pub trait Resettable: RegisterSpec {
 }
 
 /// This structure provides volatile access to registers.
+#[repr(transparent)]
 pub struct Reg<REG: RegisterSpec> {
     register: vcell::VolatileCell<REG::Ux>,
     _marker: marker::PhantomData<REG>,