Scalar function coverage

[taniabogatsch]

This PR covers all ...[scalar]... functions in the C API.
It also includes:

• some minor folder reorganization.
• slightly less verbose function names for the tests.
• duckdb_state returns state when registering the various scalar functions (still a big ugly...).

The following would help with building this reference extension.

• Python script to get the diff between the covered functions and the extension function struct. These are the functions that still need to be covered.
• A formatter.

I'll also throw the command to get some debugging going in here.

lldb ./configure/venv/bin/python3 -- -m duckdb_sqllogictest --test-dir test/sql  --external-extension build/debug/reference_extension_c.duckdb_extension

[samansmink]

looks great!

[samansmink]

I can look into adding this to the template/ci tools:

• formatter
• makefile recipe to run tests in debugger

The script for detecting coverage is definitely something we need, not sure how to tackle this. I see a few options:

• python parsing script that just checks presence of function name (a little hacky perhaps?
• instrumentation DuckDB side that logs C API function calls
• instrumentation Extension side that logs C API function calls

maybe its not such a bad idea to have debug builds of DuckDB have the option of logging every C API call that is made. It will probably be really slow but it might be nice to have anyway? Ofc this would depend on the logging infrastructure to be there in time

[taniabogatsch]

Great, yeah, the debugger and the formatter will be handy.

Should we also slow down our additions here until we know which functions will be stable? The scalar function API and basic data chunk handling should be fine; we'll see.

W.r.t. the logging, this is the cleanest solution. However, the Python parsing script should suffice until this repo/extension is close to completion. :)