function popcount6Bit(value) {
  return (
    (value & 1) +
    ((value >> 1) & 1) +
    ((value >> 2) & 1) +
    ((value >> 3) & 1) +
    ((value >> 4) & 1) +
    ((value >> 5) & 1)
  );
}
Compute the popcount for 6-bit integers (values 0-63). This is the
number of 1s in the binary representation of the value.

@param {number} value The value to compute the popcount for
@returns {number} The number of 1s in the binary representation of value

@private