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>

Cargo.lock

@@ -175,6 +175,7 @@ dependencies = [
  "signal-hook",
  "test_infra",
  "thiserror",
+ "tracer",
  "vm-memory",
  "vmm",
  "vmm-sys-util",
@@ -1197,6 +1198,17 @@ dependencies = [
  "syn",
 ]
 
+[[package]]
+name = "tracer"
+version = "0.1.0"
+dependencies = [
+ "libc",
+ "log",
+ "once_cell",
+ "serde",
+ "serde_json",
+]
+
 [[package]]
 name = "unicode-ident"
 version = "1.0.4"
@@ -1516,6 +1528,7 @@ dependencies = [
  "serial_buffer",
  "signal-hook",
  "thiserror",
+ "tracer",
  "uuid",
  "versionize",
  "versionize_derive",