Add projected bit fields

[rauhul]

Removes raw access to bit fields from generated Register Read and Write views. Users will now need to use the raw computed property of the Read and Write views to access bit fields as the underlying register storage integer type.

For example, if a programmer previously had a register with the following field and mutation

@Register(bitWidth: 32)
struct R32 {
  @ReadWrite(bits: 0..<1)
  var en: EN
}

r32.modify { $0.en = 1 }

The modify closure must be updated to: $0.raw.en = 1

---

Introduces the BitFieldProjectable protocol which describes how types can be encoded to or decoded from raw bit fields. Bit field macros now have an optional as: argument which take a type conforming to the BitFieldProjectable protocol and expose the field as that type. Extending the above example with this new functionality we get the following snippet:

@Register(bitWidth: 32)
struct R32 {
  @ReadWrite(bits: 0..<1, as: Bool.self)
  var en: EN
}

r32.modify { $0.en = true }

Field and projected type bit widths cannot be statically validated to match due to limitations with the Swift language. However MMIO guarantees that such uses compile to trap if the projection is used.