Writes trace logging using the current fiber's configured traceln function.

with_trace_prefix fmt fn runs fn () with a traceln that outputs fmt before each message.

The mutex used to prevent two domains writing to stderr at once.

This might be useful if you want to write to it directly yourself, e.g. for a log reporter.

default_traceln is a suitable default implementation for Eio.Std.traceln.

It writes output to stderr, prefixing each line with a "+". If __POS__ is given, it also displays the file and line number from that. It uses traceln_mutex so that only one domain's output is written at a time.

Backends should use this for Eio.Stdenv.debug.