array join: avoid strcat, speedup from O(N^2) to O(N)
array join: avoid strcat, speedup from O(N^2) to O(N)

Previously, each iteration (except the 1st) did this amount of calls:
  2x strlen(result-so-far) + 2x strlen(element) + strlen(sep)
Where except one strlen(element) they're implicit inside strcat, and
of sizes which we already know.

Now each iteration does one strlen(element), and no strcat.

The big speedup is avoiding strlen of the result so far (twice) on
each iteration - O(N^2), but the other extra 2x strlen can add up too.

Join of an array of 2000 strings of 80 chars each:
  Windows: before: 80ms, after: 2ms
  Linux:   before: 20ms, after: 2ms
Measured using Date.now()

ccxvii/mujs,ccxvii/mujs,ccxvii/mujs,ccxvii/mujs