CP-307958: Generate event RPC more directly

Use an attribute to change the name of the fields instead of serializing with one name,
and then mapping to the other.

`ministat` confirms an improvement on `rpc_of_event`:
```
    N           Min           Max        Median           Avg        Stddev
x 922     440.79126     46094.975     467.83727     563.79259     1676.0843
+ 1131     51.346821     33407.866     58.715393     115.98006     1105.4178
Difference at 95.0% confidence
        -447.813 +/- 120.966
        -79.4286% +/- 13.1488%
        (Student's t, pooled s = 1390.95)
```

Signed-off-by: Edwin Török <[email protected]>

Author: edwintorok

ocaml/xapi-types/event_types.ml

@@ -20,37 +20,15 @@ let rpc_of_op = API.rpc_of_event_operation
 let op_of_rpc = API.event_operation_of_rpc
 
 type event = {
-    id: string
-  ; ts: string
-  ; ty: string
-  ; op: op
-  ; reference: string
-  ; snapshot: Rpc.t option
+    id: string [@key "id"]
+  ; ts: string [@key "timestamp"]
+  ; ty: string [@key "class"]
+  ; op: op [@key "operation"]
+  ; reference: string [@key "ref"]
+  ; snapshot: Rpc.t option [@key "snapshot"]
 }
 [@@deriving rpc]
 
-let ev_struct_remap =
-  [
-    ("id", "id")
-  ; ("ts", "timestamp")
-  ; ("ty", "class")
-  ; ("op", "operation")
-  ; ("reference", "ref")
-  ; ("snapshot", "snapshot")
-  ]
-
-let remap map str =
-  match str with
-  | Rpc.Dict d ->
-      Rpc.Dict (List.map (fun (k, v) -> (List.assoc k map, v)) d)
-  | _ ->
-      str
-
-let rpc_of_event ev = remap ev_struct_remap (rpc_of_event ev)
-
-let event_of_rpc rpc =
-  event_of_rpc (remap (List.map (fun (k, v) -> (v, k)) ev_struct_remap) rpc)
-
 type events = event list [@@deriving rpc]
 
 type token = string [@@deriving rpc]