Rework span completion to support adding props

[KodrAus]

Closes #166
Closes #167

This PR reworks the SpanGuard type to not rely on the props type P on its resulting span. This means we can change P while the span executes, such as by pushing properties to it, without needing to rewrite its completion function.

I've also made span completion take &mut self, so you can use guards and do manual completion as well as automatic result completion without them conflicting.

[KodrAus]

Passing guard by ref mut is going to be an issue for pushing properties to a guard; we need to take it by-value to make that work, but taking by-value means you can't protect against early returns.