subscriber: fix FmtSubscriber's auto traits inheriting Cs (#2024)

Depends on #2023 

## Motivation

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

## Solution

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

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

tracing-subscriber/src/fmt/fmt_subscriber.rs

@@ -68,7 +68,7 @@ pub struct Subscriber<C, N = format::DefaultFields, E = format::Format, W = fn()
     fmt_event: E,
     fmt_span: format::FmtSpanConfig,
     is_ansi: bool,
-    _inner: PhantomData<C>,
+    _inner: PhantomData<fn(C)>,
 }
 
 impl<C> Subscriber<C> {

tracing-subscriber/src/subscribe/mod.rs

@@ -240,7 +240,7 @@
 //! impl LogConfig {
 //!     pub fn subscriber<C>(self) -> Box<dyn Subscribe<C> + Send + Sync + 'static>
 //!     where
-//!         C: tracing_core::Collect + Send + Sync,
+//!         C: tracing_core::Collect,
 //!         for<'a> C: LookupSpan<'a>,
 //!     {
 //!         // Shared configuration regardless of where logs are output to.