Add an instrument attribute for tracing function execution

[KodrAus]

For #57

This is something that would also help when using emit in tests. It would be nice to have a span attribute that was tailored for tracing function execution:

fn test_setup() {
    let _ = emit::setup().emit_to(emit_term::stdout()).init();
}

#[emit::span_fn(setup: test_setup)]
#[test]
fn my_test() {
    ..
}

The setup parameter is one we could add to emit::span that evaluates an expression before it does the internal span setup so you've got a chance to initialize emit. Then span_fn could expand to something like:

#[emit::span(setup: test_setup, "my_test()")]
#[test]
fn my_test() {

}

If the function had arguments, those could be listed in a props collection to be included:

#[emit::span_fn(args: { #[emit::as_debug] a })]
fn some_fn(a: u32, b: Uncaptured) {
    ..
}

would expand to:

#[emit::span("some_fn({#[emit::as_debug] a})")]
fn some_fn(a: u32, b: Uncaptured) {
    ..
}

It might also be nice to include some unique identifier, like a hash of the full function name, on the emitted events so you can use them to build an aggregated flamegraph.

[KodrAus]

This isn't something we'd need direct support for in emit, but could make a good external library.