ocaml: phase 5.1 twosum.ml baseline (LeetCode #1 one-pass hashmap, index sum = 5)

Walks list with List.iteri, checking if target - x is already in
the hashtable; if yes, the earlier index plus current is the
answer; otherwise record the current pair.

  twosum [2;7;11;15] 9   = (0, 1)   2+7
  twosum [3;2;4]     6   = (1, 2)   2+4
  twosum [3;3]       6   = (0, 1)   3+3

Sum of i+j over each pair: 1 + 3 + 1 = 5.

Tests Hashtbl.find_opt + add (the iter-99 cleanup), List.iteri, and
tuple destructuring on let-binding (iter 98 'let (i, j) = twosum
... in').

63 baseline programs total.

lib/ocaml/baseline/expected.json

@@ -60,6 +60,7 @@
   "zip_unzip.ml": 1000,
   "sieve.ml": 15,
   "sum_squares.ml": 385,
+  "twosum.ml": 5,
   "unique_set.ml": 9,
   "validate.ml": 417,
   "word_count.ml": 3