Enumeratum support

[cb372]

Add support for reading and writing enumeratum IntEnums. These are enumerations whose elements have both a name and an integer value.

This means that in Mu we'll be able to preserve the integer values of protobuf enums. Given a .proto file:

enum Colour {
  BLUE = 0;
  RED = 5;
}

, instead of generating:

sealed trait Colour
object Colour {
  case object BLUE extends Colour
  case object RED extends Colour
}

like we currently do, we can instead encode it as:

sealed trait Colour(val value: Int) extends IntEnumEntry
object Colour extends IntEnum[Colour] {
  case object BLUE extends Colour(0)
  case object RED extends Colour(5)

  val values = findValues
}

This means PBDirect can encode/decode the enum using the correct integer values.

Also if lloydmeta/enumeratum#263 gets merged and released, we will be able to handle enums with aliases.

[juanpedromoreno]

👍 thanks @cb372