v2: use a byte array for the characteristics bitfield

[mvdan]

Right now, characteristics is as follows:

type Characteristics struct {
	Hi uint64
	Lo uint64
}

And, as per the code, it encodes and decodes as little-endian.

If Go had a uint128 type, I'd definitely agree with this approach. For better or worse it doesn't, so using encoding/binary.LittleEndian doesn't save us complexity - we still have to choose between Hi and Lo.

I think it would be easier to use [16]byte, as proposed by @rvagg here: #239 (comment)

Then the logic should also become fairly straightforward to follow. For example, a "get bit" or "set bit" method on Characteristics would use pos/8 to select an array index, and pos%8 to select the bit within its byte.

[mvdan]

Another note: we could also define nice user-friendly APIs, such as:

type Characteristic uint8

const FooBar Characteristic = 1
// ...

func (Characteristics) Get(c Characteristic) bool { ... }
func (Characteristics) Set(c Characteristic, enabled bool) { ... }

This sorta assumes that all characteristics will be booleans, but I think that's the idea.