trace-core: Fix NoSubscriber causing panics (#975)

PR #973 changed the `tokio_trace_core::span::Id::from_u64` function to
require that the provided `u64` be greater than zero. However, I had
forgotten that the implementation of `Subscriber` for the `NoSubscriber`
type (which is used when no default subscriber is set) always returned
`span::Id::from_u64(0)` from its `new_span` method. In combination with
the assert added in #973, this means that every time a span is hit when
no subscriber is set, `tokio-trace-core` will panic.

This branch fixes the panics by having `NoSubscriber` construct span IDs
using a different (arbitrarily chosen) non-zero constant.

Signed-off-by: Eliza Weisman <[email protected]>

tokio-trace/tokio-trace-core/src/dispatcher.rs

@@ -252,7 +252,7 @@ impl Subscriber for NoSubscriber {
     }
 
     fn new_span(&self, _: &span::Attributes) -> span::Id {
-        span::Id::from_u64(0)
+        span::Id::from_u64(0xDEAD)
     }
 
     fn event(&self, _event: &Event) {}