The following changes are introduced:
- If-statements and asserts have now an op-type.
- If-statements are primitive (not encoded with switch).
- The && and || operators are implemented using IfE.
- The front-end uses better types for conditionals and asserts.

Rodolphe Lepigre authored Jul 15, 2021 and Michael Sammler committed Jul 16, 2021

This requires adding an option [--soft] to [refinedc clean] in order
to prevent the [_CoqProject] file to be edited locally when building
or cleaning the examples.