Change impl Format for core::time::Duration

[jonathanpallant]

impl Format for core::time::Duration prints "Duration {{ secs: {=u64}, nanos: {=u32} }}", duration.as_secs(), duration.subsec_nanos()

It would be nicer to have it print "{=u64:tus}", duration.as_micros() as u64, because tus prints timestamps nicely.

[jonathanpallant]

Or even {=u64}.{=u32:09} would be better.