Refactor backend module instantiation

[jeromesimeon]

It might be necessary/cleaner to instantiate the Q*cert modules for code generation before they are used in Ergo. This would freeze foreign types (e.g., for date/time) which means constructing type models is more natural. This concretely means:

• [x] Split the Compiler directory into pre-Jura Backend and post-Ergo ErgoCompiler parts
• [ ] Import the Backend instead of Qcert in the Ergo part of the code
• [x] Re-factor the built-in operators, CTO to model translation, etc to rely on the instantiated backend compiler
• [ ] Fix up of backend module types and definitions so it results in cleaner extracted code

[jeromesimeon]

A large part of this item is done in 1dfdc9d. Still further cleanup is needed and we can start to take advantage of the refactoring (for type import, standard library, etc).