trace: Allow field names to be separated by .s (#1027)

## Motivation

In order to support conventions that add namespacing to `tokio-trace`
field names, it's necessary to accept at least one type of separator
character. Currently, the `tokio-trace` macros only accept valid Rust
identifiers, so there is no clear separator character for namespaced
conventions. See also #1018.

## Solution

This branch changes the single `ident` fragment matcher for field names
to match *one or more* `ident` fragments separated by `.` characters.

## Notes

The resulting key is still exposed to `tokio-trace-core` as a string
constant created by stringifying the dotted expression. However, if
`tokio-trace-core` were later to adopt a first class notion of
hierarchical field keys, we would be able to track that change in
`tokio-trace` as an implementation detail.

Closes #1018.
Closes #1022.

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

tokio-trace/tests/span.rs

@@ -278,6 +278,23 @@ fn moved_field() {
     handle.assert_finished();
 }
 
+#[test]
+fn dotted_field_name() {
+    let (subscriber, handle) = subscriber::mock()
+        .new_span(
+            span::mock()
+                .named("foo")
+                .with_field(field::mock("fields.bar").with_value(&true).only()),
+        )
+        .done()
+        .run_with_handle();
+    with_default(subscriber, || {
+        span!(Level::TRACE, "foo", fields.bar = true);
+    });
+
+    handle.assert_finished();
+}
+
 #[test]
 fn borrowed_field() {
     let (subscriber, handle) = subscriber::mock()