refactor: better select challenges function

[vmx]

The function to select the challenges changes a lot over time and now is hardly comprehensible. Hence it's refacored into a more understandable version. It uses a div_ceil function which is expecte to be included in the Rust standard library at some point in the future.

[DrPeterVanNostrand]

Looks good to me.

Just FYI if you're interested, I believe challenges_count_all is a leftover from when PoRep's de#cluded a specific number of challenges per layer (and challenges_count_all meant the "total number of challenges across all layers per partition"). And a potential future refactor that I would also support is renaming challenges_count_all to something like partition_challenge_count.

[vmx]

Just FYI if you're interested, I believe challenges_count_all is a leftover from when PoRep's de#cluded a specific number of challenges per layer (and challenges_count_all meant the "total number of challenges across all layers per partition"). And a potential future refactor that I would also support is renaming challenges_count_all to something like partition_challenge_count.

Exactly! I tackle that in a later commit on the ni-porep work: d2505cb.

The base branch was changed.