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/Cargo.toml

@@ -0,0 +1,15 @@
+[package]
+name = "tracer"
+version = "0.1.0"
+authors = ["The Cloud Hypervisor Authors"]
+edition = "2021"
+
+[dependencies]
+libc = "0.2.126"
+log = "0.4.17"
+once_cell = "1.12.0"
+serde = { version = "1.0.137", features = ["rc", "derive"] }
+serde_json = "1.0.81"
+
+[features]
+tracing = []