feat(span): liberally derive std traits

[TheLostLambda]

Bringing in line with https://rust-lang.github.io/api-guidelines/interoperability.html#types-eagerly-implement-common-traits-c-common-traits !

Left out Display and Default, since those don't make quite as much sense here, though I'm sure someone could make a case for Default if they really found it useful.

I ran into this when I noticed Span wasn't Copy!