tracing: Tracing infrastructure

Add a new feature "tracing" that enables tracing functionality via the
"tracer" crate (sadly features and crates cannot share the same name.)

Setup: tracer::start()

The main functionality is a tracer::trace_scope()! macro that will add
trace points for the duration of the scope. Tracing events are per
thread.

Finish: tracer::end() this will write the trace file (pretty printed
JSON) to a file in the current directory.

Signed-off-by: Rob Bradford <robert.bradford@intel.com>

tracer/src/lib.rs

@@ -0,0 +1,18 @@
+// Copyright © 2022 Intel Corporation
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+
+#[cfg(feature = "tracing")]
+#[macro_use]
+extern crate log;
+
+#[cfg(not(feature = "tracing"))]
+mod tracer_noop;
+#[cfg(not(feature = "tracing"))]
+pub use tracer_noop::*;
+
+#[cfg(feature = "tracing")]
+mod tracer;
+#[cfg(feature = "tracing")]
+pub use tracer::*;