Convert SVDEnumerations into BitFieldProjections

[rauhul]

Updates SVD2Swift to generate bit field projections if the underlying
SVDField contains an SVDEnumeration. Fully closed enumerations
(where the enumeration describes all possible bit patterns) are exported
as Swift enums, and structs otherwise.

Fields with these projected types can be accessed with safer API:

@Register(bitWidth: 32)
struct ExampleRegister {
  @ReadWrite(bits: 2..<7, as: ExampleEnumeratedValues.self)
  var exampleField: ExampleField
}

var exampleRegister = ExampleRegister(...)
exampleRegister.modify { rw in
  switch rw.exampleField {
  case .namedExample0:
    print("Named pattern")
  case ._0b00010:
    print("Anonymous pattern")
  case .exampleWithDontCare:
    print("Multi-pattern: e.g. 0bXXX00")
  default:
    print("Unknown pattern")
  }
}