ext/bcmath: Improving bcpow() performance

[SakiTakamachi]

I ran 500,000 comparisons with PHP 8.3 results. (I would like to perform more comparisons, but PHP 8.3's bcpow() is too heavy and the comparisons never finish.)
No problems were detected.

Benchmarks

The speed difference is especially noticeable when base is less than 1 and there are many leading zeros.
This is because try to eliminate as many zeros as possible during the calculation, and then fill in the missing digits with zeros later.

1:

for ($i = 0; $i < 2000000; $i++) {
    bcpow('123', '4', 0);
}

Benchmark 1: /php-dev/sapi/cli/php /mount/bc/pow/1.php
  Time (mean ± σ):     244.8 ms ±   1.3 ms    [User: 235.5 ms, System: 4.2 ms]
  Range (min … max):   242.8 ms … 247.7 ms    12 runs
 
Benchmark 2: /master/sapi/cli/php /mount/bc/pow/1.php
  Time (mean ± σ):     279.4 ms ±   3.4 ms    [User: 269.1 ms, System: 5.2 ms]
  Range (min … max):   275.7 ms … 287.4 ms    10 runs
 
Summary
  '/php-dev/sapi/cli/php /mount/bc/pow/1.php' ran
    1.14 ± 0.02 times faster than '/master/sapi/cli/php /mount/bc/pow/1.php'

2:

for ($i = 0; $i < 200000; $i++) {
    bcpow('123456.789', '64', 10);
}

Benchmark 1: /php-dev/sapi/cli/php /mount/bc/pow/2.php
  Time (mean ± σ):     360.9 ms ±   1.1 ms    [User: 350.9 ms, System: 4.8 ms]
  Range (min … max):   359.4 ms … 363.2 ms    10 runs
 
Benchmark 2: /master/sapi/cli/php /mount/bc/pow/2.php
  Time (mean ± σ):     506.4 ms ±   4.3 ms    [User: 496.2 ms, System: 4.7 ms]
  Range (min … max):   501.0 ms … 514.3 ms    10 runs
 
Summary
  '/php-dev/sapi/cli/php /mount/bc/pow/2.php' ran
    1.40 ± 0.01 times faster than '/master/sapi/cli/php /mount/bc/pow/2.php'

3:

for ($i = 0; $i < 100000; $i++) {
    bcpow('0.00123456', '120', 10);
}

Benchmark 1: /php-dev/sapi/cli/php /mount/bc/pow/3.php
  Time (mean ± σ):     304.9 ms ±   2.9 ms    [User: 295.4 ms, System: 4.4 ms]
  Range (min … max):   299.9 ms … 308.7 ms    10 runs
 
Benchmark 2: /master/sapi/cli/php /mount/bc/pow/3.php
  Time (mean ± σ):     808.8 ms ±   9.0 ms    [User: 797.4 ms, System: 5.6 ms]
  Range (min … max):   799.3 ms … 827.3 ms    10 runs
 
Summary
  '/php-dev/sapi/cli/php /mount/bc/pow/3.php' ran
    2.65 ± 0.04 times faster than '/master/sapi/cli/php /mount/bc/pow/3.php'

4:

for ($i = 0; $i < 1000; $i++) {
    bcpow('0.00000000000000123456', '2000', 10);
}

Benchmark 1: /php-dev/sapi/cli/php /mount/bc/pow/4.php
  Time (mean ± σ):     433.7 ms ±   1.9 ms    [User: 422.2 ms, System: 6.2 ms]
  Range (min … max):   431.6 ms … 437.6 ms    10 runs
 
Benchmark 2: /master/sapi/cli/php /mount/bc/pow/4.php
  Time (mean ± σ):      6.024 s ±  0.071 s    [User: 6.011 s, System: 0.007 s]
  Range (min … max):    5.986 s …  6.225 s    10 runs
 
  Warning: Statistical outliers were detected. Consider re-running this benchmark on a quiet system without any interferences from other programs. It might help to use the '--warmup' or '--prepare' options.
 
Summary
  '/php-dev/sapi/cli/php /mount/bc/pow/4.php' ran
   13.89 ± 0.18 times faster than '/master/sapi/cli/php /mount/bc/pow/4.php'

[SakiTakamachi]

There were still some things I wanted to change, so I reverted it to a draft.

[SakiTakamachi]

Ready for review.
The failed tests are irrelevant.

cc: @nielsdos

[SakiTakamachi]

I force pushed by mistake.

I'll open it again once I've confirmed the code 🙏

[SakiTakamachi]

I reverted to the original code.