core/filtermaps: define APIs for map, epoch calculation

[rjl493456442]

This pull request refines the filtermap implementation, defining key APIs for map and
epoch calculations to improve readability.

This pull request doesn't change any logic, it's a pure cleanup.

[Copilot]

Copilot reviewed 9 out of 9 changed files in this pull request and generated 2 comments.

[zsfelfoldi]

I added the assertions of bitLength back in the accessor functions. It's a good change that sanitize catches invalid parameters earlier and in a nicer way but it's not good if one function assumes something about a parameter that is checked in an entirely different place and just fails silently if that assumption is not true any more for some reason (the parameter comes from elsewhere or sanitizing is not called or whatever).
Also I changed back the rowIndex vs mapRowIndex terminology. You renamed mapRowIndex to rowIndex and rowIndex to row, which is not good because mapRowIndex is not a row index, it is the combined storage position index derived from a map index and a row index. Maybe we could come up with a better name for it but that name should not be rowIndex.
Otherwise LGTM.