Implement Value for more types

[jonhoo]

tracing_core::Value is currently missing implementations for some pretty common types from the standard library. The ones I ran into:

• f32 and f64
• String — this will (I think) move the type, so it may not do what people want, but it'll give us &String. If we could implement for AsRef<str>, now that would be neat. But may not work because of coherence.
• Cow<T> where T: Value (though this may require a MSRV bump for this)

Of course, it's possible to use %/Display for these, but it'd be nice if one didn't have to.

[pickfire]

I ran into Path and PathBuf as well, supporting Display would be good.

[Veetaha]

I suggest at least starting adding an impl for String, because it is cumbersome to write all the time

let owned_string: String = ...;

info!(owned_string = onwed_string.as_str(), "foo");

// or less-efficient, but less verbose:

info!(%owned_string, "foo");

[poelzi]

Especially working with Path is very cumbersum with no support for &Path and &OsString