device: reduce RoutineHandshake allocations

device: reduce RoutineHandshake allocations

Reduce allocations by eliminating byte reader, hand-rolled decoding and
reusing message structs.

Synthetic benchmark:

    var msgSink MessageInitiation
    func BenchmarkMessageInitiationUnmarshal(b *testing.B) {
        packet := make([]byte, MessageInitiationSize)
        reader := bytes.NewReader(packet)
        err := binary.Read(reader, binary.LittleEndian, &msgSink)
        if err != nil {
            b.Fatal(err)
        }
        b.Run("binary.Read", func(b *testing.B) {
            b.ReportAllocs()
            for range b.N {
                reader := bytes.NewReader(packet)
                _ = binary.Read(reader, binary.LittleEndian, &msgSink)
            }
        })
        b.Run("unmarshal", func(b *testing.B) {
            b.ReportAllocs()
            for range b.N {
                _ = msgSink.unmarshal(packet)
            }
        })
    }

Results:
                                         │      -      │
                                         │   sec/op    │
MessageInitiationUnmarshal/binary.Read-8   1.508µ ± 2%
MessageInitiationUnmarshal/unmarshal-8     12.66n ± 2%

                                         │      -       │
                                         │     B/op     │
MessageInitiationUnmarshal/binary.Read-8   208.0 ± 0%
MessageInitiationUnmarshal/unmarshal-8     0.000 ± 0%

                                         │      -       │
                                         │  allocs/op   │
MessageInitiationUnmarshal/binary.Read-8   2.000 ± 0%
MessageInitiationUnmarshal/unmarshal-8     0.000 ± 0%

Signed-off-by: Alexander Yastrebov <yastrebov.alex@gmail.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>