change how top-level emit/asm statements work

[zerbina]

Summary

Only treat emit and asm statements at the module-scope and
outside of any complex expression as top-level emit/asm statements.

Details

The main goal is to move top-level emit/asm handling largely out of the
code generator and into the orchestrator; the user-facing behaviour
changing is a side effect.

When creating the module lifecycle routines from module AST, emit/asm
statements that are at the top-level are now extracted and put into a
separate section. They're then translated from AST to MIR and
emitted as the new bekEmit event by the backends.process iterator.

For the C backend, they're passed onto the code generator; they're not
possible for the other two backends. Section determination is removed
from cgen.

While the section modifiers are translated to the same C file section
as before, all top-level emit/asm statements are now processed before
other top-level code. Tests for top-level emit statements and those
using them are updated (where needed).

[zerbina]

I do think that emit sections are a misfeature (not only in their current incarnation, but in general), but given that they're still used by some tests without and there being a direct replacement, it was easier to just decouple the feature from cgen for now.

[saem]

/merge

[github-actions]

Merge requested by: @saem

Contents after the first section break of the PR description has been removed and preserved below:

---

Notes for Reviewers

• split out from implement a new CGIR and C code generator #1625 and improved from