Set active span on thread instead of stack

[joshwlewis]

WIP!

Alternative to #918. The idea here is to set the active span on the thread so that buildpacks can nest their own spans within the libcnb provided one - extending the waterfall trace from kodon through libcnb and into CNB specific traces.

Usage within a buildpack might look like this:

let tracer = global::tracer("my-buildpack");
tracer.in_span("download-go", |ctx| {
    let s = ctx.span();
    s.set_attribute(KeyValue::new("go_version", go_version.to_string()));
    do_the_download();
    s.add_event("downloaded")
});