Add span compression

[mikker]

See #1021

Currently passing all tests but one, which is concerning this part from the spec:

    if (!child.isCompressionEligible()) {
        if (buffered != null) {
            report(buffered) # <-- reporting buffered from sibling
            buffered = null  # <-- resetting buffer
        }
        report(child)
        return
    }

Keeping track of children and buffers and such is very difficult and the fact that we use the instrumenter to report instead of letting the spans themselves do it doesn't make it easier.

[apmmachine]

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• Reason: The PR is not allowed to run in the CI yet

• Start Time: 2022-02-03T05:45:23.157+0000

• Duration: 7 min 6 sec

• Commit: 795fe77

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[mikker]

Hey friends! Closing this one because cleaning my GH up a bit. Will leave the branch – but maybe copy it somewhere if you want to make sure to not lose the work. Hope you're all well ❤️