trace: Change Span::enter to return a guard, add Span::in_scope (#1076)

## Motivation

Currently, the primary way to use a span is to use `.enter` and pass a
closure to be executed under the span. While that is convenient in many
settings, it also comes with two decently inconvenient drawbacks:

 - It breaks control flow statements like `return`, `?`, `break`, and
   `continue`
 - It require re-indenting a potentially large chunk of code if you wish
   it to appear under a span

## Solution

This branch changes the `Span::enter` function to return a scope guard 
that exits the span when dropped, as in:
```rust
let guard = span.enter();

// code here is within the span

drop(guard);

// code here is no longer within the span
```
The method previously called `enter`, which takes a closure and 
executes it in the span's context, is now called `Span::in_scope`, and
was reimplemented on top of the new `enter` method. 

This is a breaking change to `tokio-trace` that will be part of the
upcoming 0.2 release.

Closes #1075 

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

tokio-trace/examples/counters.rs

@@ -125,7 +125,7 @@ fn main() {
 
     tokio_trace::subscriber::with_default(subscriber, || {
         let mut foo: u64 = 2;
-        span!(Level::TRACE, "my_great_span", foo_count = &foo).enter(|| {
+        span!(Level::TRACE, "my_great_span", foo_count = &foo).in_scope(|| {
             foo += 1;
             info!({ yak_shaved = true, yak_count = 1 }, "hi from inside my span");
             span!(
@@ -134,7 +134,7 @@ fn main() {
                 foo_count = &foo,
                 baz_count = 5
             )
-            .enter(|| {
+            .in_scope(|| {
                 warn!({ yak_shaved = false, yak_count = -1 }, "failed to shave yak");
             });
         });