C API: Mark as stable

[roberth]

Is your feature request related to a problem? Please describe.

The goal of the C API is to provide a stable interface. This issue gathers the final steps.

This issue depends on every other issue in the milestone.

Describe the solution you'd like

• review [c api] labeled issues
• review [c api] labeled PRs
• make sure known language bindings are up to date and ask whether they ran into interface issues
• grep for unstable C bindings in the code, and replace
• grep for C API (experimental)

Additional context

Priorities

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