Improve autoloading efficiency

The current autoloading behavior used by Polylang is brutal, it calls file_exists (which has to go out to disk) three times for each class PHP loads, regardless of whether or not the class belongs to polylang.

If you are using object-orientated libraries which could load hundreds of classes, this is a lot of unnecessary disk access.

A quick grep of the codebase tells me that you should be able to prevent this via:

if(0 !== substr_compare('PLL_', $class, 0, 4)) return;

https://wordpress.org/plugins/polylang/