Merge properties of `Raw` struct into `Read`/`Write` structs

[fumoboy007]

(I originally proposed this change here.)

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The @Register macro generates 3 structs: Read, Write, and Raw. The Raw struct is accessed through the .raw property of the Read and Write structs:

@Register(bitWidth: 32)
struct CR1 {
  @ReadOnly(bits: 0..<1, as: Bool.self)
  var en: EN
  @WriteOnly(bits: 1..<2, as: Bool.self)
  var clken: CLKEN
}

cr1.modify { (read, write) in
  print(read.raw.en)
  write.raw.clken = 1
}

One issue with this approach is that the generated Raw struct is reused by both the Read and Write structs, which means it needs to contain all properties of both structs. However, that allows a developer to write something like write.raw.en = 1, which is unsafe because en is supposed to be read-only.

Instead, I propose that the properties of the Raw struct be merged into the Read/Write structs. The new properties could be prefixed with $, similar to property wrappers, to denote that the properties are just a different view on the non-prefixed properties.

cr1.modify { (read, write) in
  print(read.$en)
  write.$clken = 1

  write.$en = 1  // compile error: `$en` does not exist
}

(If #70 is accepted, this change would also make the Read/Write structs less awkward by avoiding having similarly-named rawValue and raw properties.)