1.4.0

Update minimal required versions of the dependencies: Coq 8.13 and Mathcomp 1.12

• added behd (remove the head/minimal key+value) function for finmap
• added a class for omap-like functions on unionmaps
• expanded theory for working with natmaps as histories
• code updated according to the new requirements of dependencies: #[export] attributes, lemma renamings etc
• removed problematic setoid for funext
• removed deprecated subPCM construction