Fiat Crypto Legacy for Coq 8.16, 8.17, 8.18

This is the last version of Fiat Crypto Legacy (the S&P 2019 paper version) compatible with Coq 8.16, 8.17, 8.18. This version supports Coq versions 8.16 -- 8.18.

What's Changed

• [sp2019latest] [CI] Drop Coq < 8.16 by @JasonGross in #1691
• Bump bbv from 6144e21 to f4caa05 by @dependabot in #1690
• [sp2019latest] User docker for master test by @JasonGross in #1696

Full Changelog: SP2019+V8.15...SP2019+V8.16