circularbuffer: Add circular buffer utility struct

[FintasticMan]

Implements a simple circular buffer template. buf[0] is the value at the current index. You can use the ++ and -- operators to move the index. You can initialise the buffer using aggregate initialisation, meaning you can just initialise with a curly-brace list.

[github-actions[bot]]

Build size and comparison to main:	Section	Size	Difference
text	409268B	0B
data	996B	0B
bss	63356B	0B

[FintasticMan]

I've just pushed a change that switches NotificationManager to use this new CircularBuffer. IMO it makes the code easier to understand from a glance.

[Riksu9000]

Maybe a static sized vector class is more along the lines of what we need for NotificationManager, along with a helper function that will erase the oldest entry if pushing a new element wouldn't fit otherwise.

See #1712

[FintasticMan]

Merged with #826.