Optimize span constructor

[Jean85]

This started as an investigation over #1273. I tried to run PHPBench over the Span constructor, and this is the result applying 5f94026:

Results seems promising, but the operation seems very small to obtain a consistent bench; @Ocramius any suggestions? You can also try this branch if you want to check if there's an improvement on your profiled test suite.

[Ocramius]

@Jean85 this is a good start, and we can iterate on it after the benchmarks are committed to mainline

[Jean85]

@Ocramius do you agree that we shouldn't put this in CI? In that case, how would you generate a sensible baseline? We would have to re-run them every time manually?

[Ocramius]

@Ocramius do you agree that we shouldn't put this in CI?

TBH, having it in CI is generally good value: obviously trouble for older PHP versions, but it doesn't need to run on 7.2.

The reason for having it in CI is not to verify performance, but just to make sure that the benchmarks can still be run, even after years: otherwise, they quickly decay, and become unusable.

[Ocramius]

What you can do in CI is running with --revs=1 --iterations=1 - that makes sure that the bench is still usable.

[Jean85]

@ste93cry this is ready for review.

[stayallive]

This seems like a good idea 👍 happy to see you guys diving so deep in the performance 💪