Import global functions and constants for enhanced performance

This is better for performance because these then don't need to be reevaluated every time they are called.

When encountering an unqualified function or constant reference, PHP will first try to locate a symbol in the current namespace by that name, and then fall back to the global namespace.
This short-circuits the check, which has substantial performance effects in some cases - in particular, ord(), chr() and strlen() show ~1500x faster calls when they are fully qualified.

However, this doesn't mean that PM is getting a massive amount faster. In real world terms, this translates to about 10-15% performance improvement.
But before anyone gets excited, you should know that the CodeOptimizer in the PreProcessor repo has been applying fully-qualified symbol optimizations to Jenkins builds for years, which is one of the reasons why Jenkins builds have better performance than home-built or source installations.
We're choosing to do this for the sake of future SafePHP integration and also to be able to get rid of the buggy CodeOptimizer, so that phar and source are more consistent.

src/pocketmine/MemoryManager.php

@@ -28,6 +28,39 @@ use pocketmine\scheduler\DumpWorkerMemoryTask;
 use pocketmine\scheduler\GarbageCollectionTask;
 use pocketmine\timings\Timings;
 use pocketmine\utils\Utils;
+use function arsort;
+use function count;
+use function fclose;
+use function file_exists;
+use function file_put_contents;
+use function fopen;
+use function fwrite;
+use function gc_collect_cycles;
+use function gc_disable;
+use function gc_enable;
+use function get_class;
+use function get_declared_classes;
+use function implode;
+use function ini_get;
+use function ini_set;
+use function is_array;
+use function is_object;
+use function is_resource;
+use function is_string;
+use function json_encode;
+use function min;
+use function mkdir;
+use function preg_match;
+use function print_r;
+use function round;
+use function spl_object_hash;
+use function sprintf;
+use function strlen;
+use function strtoupper;
+use function substr;
+use const JSON_PRETTY_PRINT;
+use const JSON_UNESCAPED_SLASHES;
+use const SORT_NUMERIC;
 
 class MemoryManager{