perf(container): cache dynamic initializer instances

[xHeaven]

Dynamic initializers were resolved through the container every time an unresolved, non-singleton class was looked up. Because these initializer classes are not themselves singleton-cached, each lookup re-instantiated every registered dynamic initializer and repeated reflection/autowiring work just to call canInitialize().

Reprod

$container->addInitializer(DatabaseInitializer::class);
$container->addInitializer(CacheInitializer::class);

$container->get(Foo::class); // pass1
$container->get(Foo::class); // pass2
$container->get(Foo::class); // pass3

Before

// pass1
new DatabaseInitializer();
new CacheInitializer();

new Foo();

// pass2
new DatabaseInitializer();
new CacheInitializer();

new Foo();

// pass3
new DatabaseInitializer();
new CacheInitializer();

new Foo();

After

// pass1
new DatabaseInitializer();
new CacheInitializer();

new Foo();

// pass2
// DatabaseInitializer, CacheInitializer reused
new Foo();

// pass3
// DatabaseInitializer, CacheInitializer reused
new Foo();

[brendt]

Feel free to merge when CI is done :)

[github-actions]

Benchmark Results

Comparison of fix/dynamic-initializer-cache against 3.x (b102c7fe7513675b96bbcdc892c4fb99e49a291c).

Open to see the benchmark results

Benchmark	Set	Mem. Peak	Time	Variability
ContainerBench(benchResolveWithDynamicInitializer)	-	4.078mb +0.03%	4.548μs -42.98%	±0.66% -34.99%

_{Generated by phpbench against commit 718e711}