ocaml: phase 5.1 brainfuck.ml baseline (subset interpreter)

Five '+++++.' groups, cumulative accumulator 5+10+15+20+25 = 75.

This is a brainfuck *subset* — only > < + - . (no [ ] looping). That's
intentional: the goal is to stress imperative idioms that the recently
added Array module + array indexing syntax + s.[i] make ergonomic, all
in one program.

Exercises:
  Array.make 256 0
  arr.(!ptr)
  arr.(!ptr) <- arr.(!ptr) + 1
  prog.[!pc]
  ref / ! / :=
  while + nested if/else if/else if for op dispatch

25 baseline programs total.

lib/ocaml/baseline/expected.json

@@ -2,6 +2,7 @@
   "anagrams.ml": 3,
   "bfs.ml": 6,
   "btree.ml": 39,
+  "brainfuck.ml": 75,
   "caesar.ml": 215,
   "calc.ml": 13,
   "closures.ml": 315,