mir: implement the new control-flow primitives

[zerbina]

Summary

Implement the goto-based control-flow primitives for the MIR -- as a consequence, destructor injection has to happen in mirgen already. The MIR is now similar/versatile enough to the CGIR to replace the latter.

Details

... to be written ...

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To-Do

• [x] update the pretty-printer
• [ ] implement the actual destructor injection
• [ ] implement a pass for eliminating unnecessary destroy operations
• [ ] (maybe) implement a pass for eliminating empty finally sections
• [ ] update the now-working tests
• [ ] write a proper commit message

Notes for Reviewers

• wraps up the control-flow rework of the MIR
• wraps up the MIR rework as sketched out here