ocaml: phase 5 HM let-rec + cons / append op types (+6 tests, 357 total)

ocaml-infer-let-rec pre-binds the function name to a fresh tv before
inferring rhs (which may recursively call the name), unifies the
inferred rhs type with the tv, generalizes, then infers body.

Builtin env types :: : 'a -> 'a list -> 'a list and @ : 'a list ->
'a list -> 'a list — needed because :op compiles to (:app (:app (:var
OP) L) R) and previously these var lookups failed.

Examples now infer:
  let rec fact n = if ... in fact : Int -> Int
  let rec len lst = ... in len    : 'a list -> Int
  let rec map f xs = ... in map   : ('a -> 'b) -> 'a list -> 'b list
  1 :: [2; 3]                      : Int list
  let rec sum lst = ... in sum [1;2;3] : Int

Scoreboard refreshed: 358/358 across 14 suites.

lib/ocaml/scoreboard.json

@@ -1,21 +1,21 @@
 {
   "suites": {
-    "eval-core": {"pass": 48, "fail": 0},
+    "eval-core": {"pass": 50, "fail": 0},
     "let-and": {"pass": 3, "fail": 0},
-    "misc": {"pass": 52, "fail": 0},
-    "parser": {"pass": 87, "fail": 0},
+    "misc": {"pass": 61, "fail": 0},
+    "parser": {"pass": 93, "fail": 0},
     "phase1-params": {"pass": 2, "fail": 0},
     "phase2-exn": {"pass": 8, "fail": 0},
     "phase2-function": {"pass": 3, "fail": 0},
     "phase2-loops": {"pass": 4, "fail": 0},
     "phase2-refs": {"pass": 6, "fail": 0},
-    "phase3-adt": {"pass": 19, "fail": 0},
+    "phase3-adt": {"pass": 24, "fail": 0},
     "phase4-modules": {"pass": 12, "fail": 0},
-    "phase5-hm": {"pass": 19, "fail": 0},
-    "phase6-stdlib": {"pass": 39, "fail": 0},
+    "phase5-hm": {"pass": 31, "fail": 0},
+    "phase6-stdlib": {"pass": 43, "fail": 0},
     "tokenize": {"pass": 18, "fail": 0}
   },
-  "total_pass": 320,
+  "total_pass": 358,
   "total_fail": 0,
-  "total": 320
+  "total": 358
 }