ocaml: phase 5.1 base_n.ml baseline (int to base-N string, length sum = 17)

36-character digit alphabet '0..9A..Z' supports any base 2..36. Loop
divides the magnitude by base and prepends the digit:

  while !m > 0 do
    acc := String.make 1 digits.[!m mod base] ^ !acc;
    m := !m / base
  done

Special-cases n = 0 -> '0' and prepends '-' for negatives.

Test cases (length, since the strings differ in alphabet):
  255 hex      'FF'              2
  1024 binary  '10000000000'    11
  100 dec      '100'             3
  0 any base   '0'               1
  sum                            17

Combines digits.[i] (string indexing) + String.make 1 ch + String
concatenation in a loop.

61 baseline programs total.

lib/ocaml/baseline/expected.json

@@ -8,6 +8,7 @@
   "bigint_add.ml": 28,
   "bits.ml": 21,
   "balance.ml": 3,
+  "base_n.ml": 17,
   "bfs.ml": 6,
   "btree.ml": 39,
   "brainfuck.ml": 75,