Minimal benchmarks around Uuid::fromString(), Uuid::fromBytes(), Uuid#toString(), Uuid#getBytes()

This is mostly to store an initial state, before we go on and start manipulating the UUID library internals.

phpbench.json

@@ -0,0 +1,10 @@
+{
+    "bootstrap":       "vendor/autoload.php",
+    "path":            "tests/benchmark",
+    "retry_threshold": 5,
+    "reports": {
+        "report": {
+            "extends": "aggregate"
+        }
+    }
+}