Instrument function calls to debug-log arguments and return values

When debugging, it's currently very difficult to know the function flow even from our debug logs.

Ideally, every function call would log its input arguments and return values, even better - without being specifically instrumented. There are some [compiler features](https://gcc.gnu.org/onlinedocs/gcc/Instrumentation-Options.html#index-finstrument-functions) for this, but difficult and may be overkill.

We might need to add a `trace` logging level for this.

Start by reviewing the existing debug logging approach and the linked GCC function-instrumentation options. The issue does not name files or tests; work is complete only after the project agrees on a scoped way to trace function arguments and return values, including whether a new trace logging level is needed.

Written by the indexing model from the issue text.