Add support for generating Packable conformance using attached macros

[mattcox]

Similar to automatic conformance for Codable, Pack should be able to generate automatic conformance to the Packable protocol using attached macros.

For example, the following type:

struct Color {
    let name: String
    var red: Double
    var green: Double
    var blue: Double
    var alpha: Double = 1.0
}

Should be able to attach a macro such as @packable to the structure, and the implementation would be automatically generated.

@packable
struct Color {
    let name: String
    var red: Double
    var green: Double
    var blue: Double
    var alpha: Double = 1.0

// - - - - - - - - - - - - - - Generated - - - - - - - - - - - - - -
    init(from unpacker: inout Unpacker) throws {
        self.name = try unpacker.unpack(String.self, using: .utf16)
        self.red = try unpacker.unpack(Double.self)
        self.green = try unpacker.unpack(Double.self)
        self.blue = try unpacker.unpack(Double.self)
        self.alpha = try unpacker.unpack(Double.self)
    }

    func pack(to packer: inout Packer) throws {
        try packer.pack(name, using: .utf16)
        try packer.pack(red)
        try packer.pack(green)
        try packer.pack(blue)
        try packer.pack(alpha)
    }
// - - - - - - - - - - - - - - Generated - - - - - - - - - - - - - -
}