Integrate the certified optimizer

This MR:

• adapts the code to the branches of mi-cho-coq!48 (merged) and mi-cho-coq!53 (merged)
• replaces the optimizer in michocomp.v by the certified one of MR mi-cho-coq!53 (merged)

Note: once mi-cho-coq!53 (merged) is merged on Mi-Cho-Coq's dev branch, the changes to the CI config should be reverted.

Solves #12.