Switch to C backend

[thomasvergne]

This issue aims at transforming Bonzai to provide a C backend implementation. This would change the followings things:

• Easier runtime tests: because code would depend on C compiler
• Less easy backend: this feature would imply compiler updates such as new passes
• Better performance: no VM overhead

Considering this, the following passes should be added:

• Typed conversion from High Level IR to Typed Medium-Level IR
• Actor conversion, to convert actors, spawn, and event sending to their low-level representation: Partially implemented
• Monomorphization, to "remove" generic data types and functions
• Typed closure conversion, to remove every in-scope lambdas
• Typed closure hoisting, to move lambdas to top-level
• ANF conversion, to simplify expression representation and remove complex, or error-friendly structures
• CLang CFG conversion, to transform ternary expressions into statements
• CLang code generation

[thomasvergne]

Thoughts on native backend

While the Clang backend gives the ability to maximize performances for a program, it results that the compiler may become a bit less secure, at least for a moment.

Switching back to the virtual machine may let us develop more reliable software, and handling some features better.

Benefits of switching to VM

• Better usability in terms of developing modules
• Potentially better error handling
• Easier interoperability with native components (for example generic printing)

Drawbacks of using a VM for Bonzai

• Lower performances for a single program compared to the native one (cost of the VM overlay).
• Need to add extra steps to potentially verify the byte code outputted by the compiler