Function default_epoch_us_fn 

pub fn default_epoch_us_fn() -> Option<fn() -> u64>

The default wall clock, or None when this build has neither a platform port nor a host to ask.

phase-359 W10 — the symmetric twin of default_clock_us_fn. The predicate “does this build have a wall clock” used to be spelled at the Executor struct literal, in two arms, duplicating the cfgs that already decide whether default_epoch_us exists at all. One place answers it now.