Add RawRepresentable conformance to the generated Read and Write structs

[fumoboy007]

(I originally proposed this change here.)

---

The raw value of the register can be read/written to by accessing the .raw.storage property like so:

cr1.modify { cr1 in
  cr1.raw.storage |= (1 & CR1.RST.bitMask) << CR1.RST.bitOffset
}

This seems similar to the rawValue property of the RawRepresentable protocol. Instead of .raw.storage, perhaps the generated Read/Write structs could conform to RawRepresentable. It probably doesn’t make a functional difference but it might reduce the ramp-up time for developers who are new to the package.

cr1.modify { cr1 in
  cr1.rawValue |= (1 & CR1.RST.bitMask) << CR1.RST.bitOffset
}

(There was a concern about the optionality of the init(rawValue:) initializer; however, a non-optional initializer can satisfy an optional initializer protocol requirement.)