make os_version a Cow as it's only heap allocated sometimes

[pjenvey]

Hey @hhatto,

I played around w/ making os_version a Cow since it's usually a &'a str and only sometimes a String. I don't see any wins in cargo bench but admittedly I haven't looked at these benchmarks, I don't know if they execute any paths that would show differences.

I suspect this change isn't even worth making: it's probably a negligible win if anything, and maybe even a wash as Cow may introduce some dispatch overhead on its own (being an enum).

I figured I'd bring it here for you if you have the time to look into it further (unfortunately I don't) and find any worth to it, otherwise feel free to reject this PR as a failed experiment =]

[hhatto]

@pjenvey

Sorry for late reply.
Looks Good To Me 👍
I will merge this change.

Thanks