Cancellation for multiplication on `nat`.

Coq's stdlib has these lemmas for Z, but those for nat are missing. We use the naming scheme of Coq's stdlib.

changed title from Robbert/mul reg to {+Cancellation for multiplication on nat.+}

changed the description

enabled an automatic merge when the pipeline for 384b36be succeeds

aborted the automatic merge because source branch was updated

added 2 commits

• 25964e6d - Cancellation lemmas for multiplication on `nat`.
• 1003c61b - CHANGELOG entry.

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enabled an automatic merge when the pipeline for 1003c61b succeeds

merged

mentioned in commit f6b14fe6

PS: Reported the missing stdlib lemmas at https://github.com/coq/coq/issues/17354