BCMath: Avoid using the heap for temporary objects by using arena allocations

[nielsdos]

Benchmark: #14132

Bench 1:

Benchmark 1: ./sapi/cli/php 1.php
  Time (mean ± σ):     401.4 ms ±   2.4 ms    [User: 397.9 ms, System: 3.2 ms]
  Range (min … max):   399.2 ms … 407.8 ms    10 runs
 
Benchmark 2: ./sapi/cli/php_old 1.php
  Time (mean ± σ):     450.6 ms ±   8.0 ms    [User: 448.0 ms, System: 2.2 ms]
  Range (min … max):   445.1 ms … 472.8 ms    10 runs

Summary
  ./sapi/cli/php 1.php ran
    1.12 ± 0.02 times faster than ./sapi/cli/php_old 1.php

Bench 2:

Benchmark 1: ./sapi/cli/php 2.php
  Time (mean ± σ):     462.0 ms ±   4.5 ms    [User: 459.8 ms, System: 1.9 ms]
  Range (min … max):   454.2 ms … 467.6 ms    10 runs
 
Benchmark 2: ./sapi/cli/php_old 2.php
  Time (mean ± σ):     521.8 ms ±   8.7 ms    [User: 519.8 ms, System: 1.5 ms]
  Range (min … max):   515.1 ms … 536.5 ms    10 runs
 
Summary
  ./sapi/cli/php 2.php ran
    1.13 ± 0.02 times faster than ./sapi/cli/php_old 2.php

Bench 3:

Benchmark 1: ./sapi/cli/php 3.php
  Time (mean ± σ):     562.3 ms ±   4.9 ms    [User: 560.9 ms, System: 0.6 ms]
  Range (min … max):   557.7 ms … 571.9 ms    10 runs
 
Benchmark 2: ./sapi/cli/php_old 3.php
  Time (mean ± σ):     607.9 ms ±   9.4 ms    [User: 604.8 ms, System: 2.5 ms]
  Range (min … max):   602.9 ms … 634.0 ms    10 runs

Summary
  ./sapi/cli/php 3.php ran
    1.08 ± 0.02 times faster than ./sapi/cli/php_old 3.php