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/Server.php

@@ -106,6 +106,74 @@ use pocketmine\utils\Terminal;
 use pocketmine\utils\TextFormat;
 use pocketmine\utils\Utils;
 use pocketmine\utils\UUID;
+use function array_filter;
+use function array_key_exists;
+use function array_shift;
+use function array_sum;
+use function asort;
+use function assert;
+use function base64_encode;
+use function bin2hex;
+use function class_exists;
+use function count;
+use function define;
+use function explode;
+use function extension_loaded;
+use function file_exists;
+use function file_get_contents;
+use function file_put_contents;
+use function filemtime;
+use function floor;
+use function function_exists;
+use function gc_collect_cycles;
+use function get_class;
+use function getmypid;
+use function getopt;
+use function gettype;
+use function implode;
+use function ini_get;
+use function ini_set;
+use function is_array;
+use function is_bool;
+use function is_dir;
+use function is_object;
+use function is_string;
+use function is_subclass_of;
+use function json_decode;
+use function max;
+use function microtime;
+use function min;
+use function mkdir;
+use function pcntl_signal;
+use function pcntl_signal_dispatch;
+use function preg_replace;
+use function random_bytes;
+use function realpath;
+use function register_shutdown_function;
+use function rename;
+use function round;
+use function scandir;
+use function sleep;
+use function spl_object_hash;
+use function sprintf;
+use function str_repeat;
+use function str_replace;
+use function stripos;
+use function strlen;
+use function strrpos;
+use function strtolower;
+use function substr;
+use function time;
+use function touch;
+use function trim;
+use const DIRECTORY_SEPARATOR;
+use const PHP_EOL;
+use const PHP_INT_MAX;
+use const PTHREADS_INHERIT_NONE;
+use const SCANDIR_SORT_NONE;
+use const SIGHUP;
+use const SIGINT;
+use const SIGTERM;
 
 /**
  * The class that manages everything