bip138: clarify that 0x00 ends payload parsing

Specify that parsing stops immediately. Add a vector with an item followed
by a terminator and ignored bytes.

Clarify that TYPE 0x00 also doesn't require TYPE_LENGTH.

bip-0138/test_vectors/payload.json

@@ -0,0 +1,13 @@
+[
+  {
+    "description": "Bytes after terminator are ignored",
+    "valid": true,
+    "payload": "030001610080ff",
+    "items": [
+      {
+        "type": 3,
+        "content": "61"
+      }
+    ]
+  }
+]