Update the page swapping so no scratch space is required

[diondokter]

Suggestion by Dirbaio:

There's one trick to swap without needing scratch space, just needs slot B to be 1 page bigger.

[ABC] [abc.]
[ABC] [abcC]
[ABc] [abcC]
[ABc] [abBC]
[Abc] [abBC]
[Abc] [aABC]
[abc] [aABC]
[abc] [.ABC]

Needs "moving" each page just once, and each page gets written just once (so if your flash can do 10k erases you can do 10k swaps, with scratch you can do less because you're "concentrating" many erases into the scratch pages).
After swap slot B is "shifted" by 1, so you need to do the "unswap" in the reverse order to account for that.