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/README.md

@@ -56,7 +56,7 @@ to indicate that some code takes place within the context of that `Span`:
 ```rust
 // Construct a new span named "my span".
 let mut span = span!("my span");
-span.enter(|| {
+span.in_scope(|| {
     // Any trace events in this closure or code called by it will occur within
     // the span.
 });
@@ -88,7 +88,7 @@ use tokio_trace::field;
 pub fn shave_the_yak(yak: &mut Yak) {
     // Create a new span for this invocation of `shave_the_yak`, annotated
     // with  the yak being shaved as a *field* on the span.
-    span!("shave_the_yak", yak = field::debug(&yak)).enter(|| {
+    span!("shave_the_yak", yak = field::debug(&yak)).in_scope(|| {
         // Since the span is annotated with the yak, it is part of the context
         // for everything happening inside the span. Therefore, we don't need
         // to add it to the message for this event, as the `log` crate does.