subscriber: fix FmtLayer's auto traits inheriting S's (tokio-rs#2024

)

Depends on tokio-rs#2023

## Motivation

Currently, `FmtLayer` holds a `PhantomData<S>` with the `Subscriber`
type it wraps. This is unfortunate because it means that the
layer's auto traits such as `Send`, `Sync`, and `'static` (as well
as `UnwindSafe`, `Unpin`, etc) depend on the `Subscriber` type's auto
traits...but the layer will never actually _store_ a value of type
`S`. While all `Subscriber` will be `Send + Sync + 'static` in practice,
the `PhantomData` means that functions returning a boxed `FmtLayer`
must unnecessarily bound the subscriber type parameter.

## Solution

This commit changes the `PhantomData` to `PhantomData<fn(S)>`, solving
the problem.

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

tracing-subscriber/src/fmt/fmt_layer.rs

@@ -70,7 +70,7 @@ pub struct Layer<
     fmt_event: E,
     fmt_span: format::FmtSpanConfig,
     is_ansi: bool,
-    _inner: PhantomData<S>,
+    _inner: PhantomData<fn(S)>,
 }
 
 impl<S> Layer<S> {

tracing-subscriber/src/layer/mod.rs

@@ -236,10 +236,10 @@
 //! }
 //!
 //! impl LogConfig {
-//!     pub fn layer<C>(self) -> Box<dyn Layer<C> + Send + Sync + 'static>
+//!     pub fn layer<S>(self) -> Box<dyn Layer<S> + Send + Sync + 'static>
 //!     where
-//!         C: tracing_core::Subscriber + Send + Sync,
-//!         for<'a> C: LookupSpan<'a>,
+//!         S: tracing_core::Subscriber,
+//!         for<'a> S: LookupSpan<'a>,
 //!     {
 //!         // Shared configuration regardless of where logs are output to.
 //!         let fmt = tracing_subscriber::fmt::layer()