Use array lookup instead of recursion in Context

[Nevay]

Prevents leaking memory if Context::with() is called iteratively; following example throws an out of memory error with the recursive implementation:

$contextKey = Context::createKey('-');
$context = Context::getCurrent();
for ($i = 0;; $i++) {
    $context = $context->with($contextKey, $i);
}

[codecov]

Codecov Report

Merging #821 (3fc150f) into main (ad3ccd8) will increase coverage by 0.21%.
The diff coverage is 85.00%.

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #821      +/-   ##
============================================
+ Coverage     82.82%   83.04%   +0.21%     
+ Complexity     1829     1828       -1     
============================================
  Files           223      222       -1     
  Lines          4723     4725       +2     
============================================
+ Hits           3912     3924      +12     
+ Misses          811      801      -10     

Flag	Coverage Δ
7.4	83.06% <85.00%> (+0.22%)	⬆️
8.0	83.08% <85.00%> (+0.21%)	⬆️
8.1	83.08% <85.00%> (+0.21%)	⬆️

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Impacted Files	Coverage Δ
src/API/Trace/AbstractSpan.php	0.00% <0.00%> (ø)
src/Context/Context.php	88.88% <87.50%> (+8.88%)	⬆️
src/API/Baggage/Baggage.php	83.87% <100.00%> (ø)
src/Context/ContextKeys.php	100.00% <100.00%> (ø)

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