remove nix on 8.18

fix Makefile and opam files

Update install instructions for Coq 8.17

Update INSTALL.md

Fix float ops (MetaRocq#768)

* Remove extraction hack for hexadecimalString/decimalString
Fix string_of_float

* Fix equality decision on primitive values

Fix float ops 8.15 (MetaRocq#769)

* Support for printing floating point primitive values (MetaRocq#767)

* Fix defs in PCUICPrimitive

Merge pull request MetaRocq#770 from MetaCoq/fix-float-ops-8.14

Fix float ops

Remove old dependency graphs

Opam files for v1.1+8.16

Merge pull request MetaRocq#761 from MetaCoq/port-globenv-changes-8.15

Port globenv changes 8.15