Feature request: NonZero version of minstant::Instant

[hatoo]

I want to reduce the size of timing data using minstant, it is helpful if the size of Optionminstant::Instant is 8 byte.
So I want NonZero version of minstant::Instant although I don't know it's actually possible.

Thanks.

[ti-chi-bot]

Welcome @hatoo! It looks like this is your first issue to tikv/minstant 🎉