A struct to keep a record of a changing variable:
Keeps N_MOST_RECENT exact previous values.
For older entries, keeps a total as CUM, which together with the total amount of tests can be
used to reconstruct an average.
The BUF type is the type that is kept inside a buffer; it has multiple instances, so it’s
worth making it a little smaller, such as u16
The CUM type contains the sum of all of them, so you might want to have a bigger type, such
as u64.