Add new type similar to AtLeastOne to enforce min/max size constraints and use it instead of Vecs in AdjustedDifficulty

[arya2]

Motivation

There's repetitive code around taking a constant number of items from an iterator to collect into a Vec, and there are max size constraints that aren't currently enforced in AdjustedDifficulty methods.

It would be easier to maintain and verify correctness if we had a struct that handled this logic instead.

Design

Create a new type similar to AtLeastOne with a const generic for max size, which:

• Calls take() to ensure the max size is enforced, and

Either:

• Uses AtLeastOne as the inner type to check that there's at least one element, or
• Accepts another const generic for min size and returns an error if there are fewer items in the iterator than the min size (optional)

Currently the minimum length for this use case is 1, so using AtLeastOne as the inner type would suffice.