Pass paramaters into a new span in `#[trace]` macro

[hawkw]

copied from hawkw/tokio-trace-prototype#53:

As @hawkw said on #16:

For future work: it would be really cool if we could pass all the params to the function as fields here as well.
Since some parameters may not be valid span fields, we might consider adding an additional attribute (#[trace_ignore]?) to annotate those fields that should be skipped.

[hawkw]

We can probably just field::debug most function args, although it might still be worth having an ignore attribute for those that don't implement Debug.