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/utils/MainLogger.php

@@ -26,6 +26,33 @@ namespace pocketmine\utils;
 use LogLevel;
 use pocketmine\Thread;
 use pocketmine\Worker;
+use function fclose;
+use function fopen;
+use function fwrite;
+use function get_class;
+use function is_resource;
+use function preg_replace;
+use function sprintf;
+use function time;
+use function touch;
+use function trim;
+use const E_COMPILE_ERROR;
+use const E_COMPILE_WARNING;
+use const E_CORE_ERROR;
+use const E_CORE_WARNING;
+use const E_DEPRECATED;
+use const E_ERROR;
+use const E_NOTICE;
+use const E_PARSE;
+use const E_RECOVERABLE_ERROR;
+use const E_STRICT;
+use const E_USER_DEPRECATED;
+use const E_USER_ERROR;
+use const E_USER_NOTICE;
+use const E_USER_WARNING;
+use const E_WARNING;
+use const PHP_EOL;
+use const PTHREADS_INHERIT_NONE;
 
 class MainLogger extends \AttachableThreadedLogger{