factor out and test shim, add fixtures

Author: slevang

fixture/pcodec/codec.06/config.json

@@ -0,0 +1,7 @@
+{
+    "delta_encoding_order": 2,
+    "equal_pages_up_to": 262144,
+    "id": "pcodec",
+    "level": 8,
+    "mode_spec": "auto"
+}