Sets this dispatch as the global default for the duration of the entire program.
Will be used as a fallback if no thread-local dispatch has been set in a thread
Can only be set once; subsequent attempts to set the global default will fail.
Err if the global default has already been set.
Warning: In general, libraries should not call
set_global_default()! Doing so will cause conflicts when executables that depend on the library try to set the default collector later.