trace: Add shorthand syntax for local fields (#1103)

## Motivation

A common pattern in `tokio-trace` is to use the value of a local
variable as a field on a span or event. Currently, this requires code
like:
```rust
info!(foo = foo);
```
which is not particularly ergonomic given how commonly this occurs.
Struct initializers support a shorthand syntax for fields where the name
of the field is the same as a local variable, and `tokio-trace` should
as well.

## Solution

This branch adds support for syntax like
```rust
let foo = ...;
info!(foo);
```
and 
```rust
let foo = Foo {
    bar: ...,
    ...
};
info!(foo.bar)
```
to the `tokio-trace` span and event macros. This syntax also works with
the `Debug` and `Display` field shorthand.

The span macros previously used a field name with no value to indicate 
an uninitialized field. A new issue, #1138, has been opened for finding a
replacement syntax for uninitialized fields. Until then, the `tokio-trace` 
macros will no longer provide a way to create fields without values, 
although the `-core` API will continue to support this.

Closes #1062 

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

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

@@ -42,7 +42,7 @@ fn test_always_log() {
     info!(message = "hello world;", thingy = 42, other_thingy = 666);
     last(&a, "hello world; thingy=42 other_thingy=666");
 
-    let mut foo = span!(Level::TRACE, "foo");
+    let foo = span!(Level::TRACE, "foo");
     last(&a, "foo;");
     foo.in_scope(|| {
         last(&a, "-> foo");
@@ -55,11 +55,13 @@ fn test_always_log() {
     span!(Level::TRACE, "foo", bar = 3, baz = false);
     last(&a, "foo; bar=3 baz=false");
 
-    let mut span = span!(Level::TRACE, "foo", bar, baz);
-    span.record("bar", &3);
-    last(&a, "foo; bar=3");
-    span.record("baz", &"a string");
-    last(&a, "foo; baz=\"a string\"");
+    // TODO(#1138): determine a new syntax for uninitialized span fields, and
+    // re-enable these.
+    // let span = span!(Level::TRACE, "foo", bar = _, baz = _);
+    // span.record("bar", &3);
+    // last(&a, "foo; bar=3");
+    // span.record("baz", &"a string");
+    // last(&a, "foo; baz=\"a string\"");
 }
 
 fn last(state: &State, expected: &str) {