BCmath pow improvements

[jorgsowa]

This PR improves the performance of the BCMath function bcpow.

We don't need to perform the usual multiplication when we raise a number to a specific set of numbers. I modified the content of bc_mulitply and removed all the redundant operations for the same number.

For small numbers, there is neglectable improvement, but for bigger there is up to 5%. I'm not happy with this benchmark and I need to use something else.

Benchmark:

<?php

$numbers = ['10', '2.1234567', '2'];
$exponents = [1, 2, 10, 64, 100];

foreach($numbers as $number) {
    foreach($exponents as $exponent) {
        echo "Number: $number, Exponent: $exponent = ";
        $start = microtime(true);
        for ($i = 0; $i < 1000000; $i++) {
            bcpow($number, $exponent, 20);
        }
        echo microtime(true) - $start . PHP_EOL;
    }
}

Number	Exponent	Before	After	Difference
10	1	0.369961977005	0.3674750328064	-0.67 %
10	2	0.39027285575867	0.38206386566162	-2.14 %
10	10	0.73798298835754	0.70764207839966	-4.28 %
10	64	1.6131219863892	1.4904029369354	-8.23 %
10	100	2.5796089172363	2.4571402072906	-4.98 %
2.1234567	1	0.36139988899231	0.36435699462891	+0.82 %
2.1234567	2	0.448086977005	0.44263195991516	-1.23 %
2.1234567	10	1.6505491733551	1.5602447986603	-5.78 %
2.1234567	64	6.6499609947205	6.3584640026093	-4.58 %
2.1234567	100	14.168872833252	13.804186105728	-2.64 %
2	1	0.33516597747803	0.32968902587891	-1.66 %
2	2	0.38029599189758	0.36791205406189	-3.36 %
2	10	0.59442806243896	0.5603358745575	-6.08 %
2	64	0.94932103157043	0.86743497848511	-9.44 %
2	100	1.4967222213745	1.39244389534	-7.48 %