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/level/format/io/leveldb/LevelDB.php

@@ -33,12 +33,31 @@ use pocketmine\level\generator\GeneratorManager;
 use pocketmine\level\Level;
 use pocketmine\level\LevelException;
 use pocketmine\nbt\LittleEndianNBTStream;
-use pocketmine\nbt\tag\{
-	ByteTag, CompoundTag, FloatTag, IntTag, LongTag, StringTag
-};
+use pocketmine\nbt\tag\{ByteTag, CompoundTag, FloatTag, IntTag, LongTag, StringTag};
 use pocketmine\network\mcpe\protocol\ProtocolInfo;
 use pocketmine\utils\Binary;
 use pocketmine\utils\BinaryStream;
+use function array_values;
+use function chr;
+use function defined;
+use function explode;
+use function extension_loaded;
+use function file_exists;
+use function file_get_contents;
+use function file_put_contents;
+use function is_array;
+use function is_dir;
+use function mkdir;
+use function ord;
+use function pack;
+use function rtrim;
+use function strlen;
+use function substr;
+use function time;
+use function trim;
+use function unpack;
+use const INT32_MAX;
+use const LEVELDB_ZLIB_RAW_COMPRESSION;
 
 class LevelDB extends BaseLevelProvider{