Add turtle.getEquipLeft() and turtle.getEquipRight()

[JakobDev]

This functions allows you to get to equipped Item.

I know, this is already possible with unequip the Item, get Item Detail and reequip it. But the current methode has 2 Problems in my Opinion:

1. The User see, that the Item is unequip and reequip.
2. It will not work, if the Turtle had a full Inventory.

[SquidDev]

There's a bit of me which doesn't like leaking "internal" data such as upgrade IDs. I guess we already do this with speaker.playSound() and turtle.inspect() though so it's not the end of the world.

If we are adding this, would it also be worth adding such a method to the pocket API? This way we can keep the two libraries mostly in sync.

It might also be nice to have a program called something like upgrades which acts similarly to the existing peripherals: printing out the currently available upgrades.