TRACE issues and potential improvements

Hello, it'd be cool to have a way to replace the tracing function, like most other impls. SBCL's got :report and CCL :before/:after, for example.

My personal use case is that I want to monitor all OPEN calls in a region. I guess I could bind *TRACE-OUTPUT* and serialize to/from string, but that's a bit dirty.

Manual for reference: https://ecl.common-lisp.dev/static/manual/Environment.html#index-trace