Generated PHP files always have native type hints stripped

[JackNoordhuis]

Currently, haxe always strips the PHP native type hints and generates phpdoc. This is undesirable due to the pocketmine API requiring native type hints for some classes and methods.

We will need a macro of some sort for telling the haxe transpiler which extern methods should retain their native php type hints. Without this, it is impossible to write direct externs for the pocketmine source and a weakly typed wrapper would be needed.