Introduce a simple cache and build a new ReferenceContext only when needed

[marcelthole]

hey, maybe this could work and improve the perfomance for the use case in #156.

Here is a simple repo script:

<?php

require 'vendor/autoload.php';

$start = hrtime(true);
$openapi = \cebe\openapi\Reader::readFromYamlFile(__DIR__ . '/index.yaml');
$end = hrtime(true);

var_dump(number_format(($end - $start) / 1e6,4, ',', ' ') . ' ms');

and this are the before and after results
(left after the change, right before)

[cebe]

this could probably cause issues when the library is used to read multiple different OpenAPI descriptions in the same process. Ideally the cache should be bound to the context like we have it with ReferenceContextCache.

[marcelthole]

yeah you are right. I used the ReferenceContextCache.
But this reduced the perfomance of the first variant. The first version only used 1.3 Million times this method, the current version 4.3 Million times. But it's still better than the original version with 9.8 Million function calls