Code timing instrumentation

I think it would be nice to have a feature that when activated, will print out the timings for various sub-procedures. A nice standardised format for printing the name of the function and the line numbers could be nice.

When the feature is not activated, this code is not compiled.

The thing I find most annoying is uninstrumenting code, only to discover I want to look at some sub function timings, and I have to reinstrument the code...

The issue names no files, tests, or entry points, so first survey the Rust crate structure and existing feature or instrumentation patterns. Define the scope for opt-in timing, standardized function and line output, compile-time exclusion when disabled, and access to nested procedure timings without reinstrumenting callers.

Written by the indexing model from the issue text.