Lesson 215 should reuse memory using pool

[thejhh]

It would probably be better to use memory pool like this:

// devicePool reuses Device instances to reduce allocations.
var devicePool = sync.Pool{
	New: func() any {
		return new(Device)
	},
}

...and using something it like this:

	device := devicePool.Get().(*Device)
	device.Id = 1
	device.Mac = "EF-2B-C4-F5-D6-34"
	device.Firmware = "2.1.5"

	renderJSON(w, device, 200)

	// Optionally reset the device state
	device.Id = 0
	device.Mac = ""
	device.Firmware = ""

	// And return it to the pool.
	devicePool.Put(device)

Go has quite a good benchmark tools also, so using those would probably detect where the bottleneck is.

PS: For security reasons I'm not going to run anything from this branch on my development system.