find index using a comparator function

The guaranteed performance is in O(log n) comparisons.

result is the index where cmp results in 0 or nil if no such index
was found in this array. In case several instance equal match,
the index of one matching key will be returned, but is not
specified which one.

NYI: CLEANUP: find better name

post
  match result
    i i64 => cmp sorted_array.this[i] = equal
    nil   => true