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/test-log-support/tests/log_no_trace.rs

@@ -44,7 +44,7 @@ fn test_always_log() {
 
     let mut foo = span!(Level::TRACE, "foo");
     last(&a, "foo;");
-    foo.enter(|| {
+    foo.in_scope(|| {
         last(&a, "-> foo");
 
         trace!({foo = 3, bar = 4}, "hello {};", "san francisco");