Adds typehints compatibility

[michalbundyra]

User code and 3rd party libraries still use legacy typehints. If we try to load new class and corresponding legacy class was not loaded before then we can get error of typehints incompatibility. On loading new class we need then create alias to legacy class so PHP interpreter thinks that it's the same class and we can have seamless migration.

TODO:

• Add reverse mapping for all namespaces
• Change autoloader to use above reverse mapping
• Add tests for all cases (reverse mapping)

[weierophinney]

Combine the statements, and return early:

if (strpos($class, 'Expressive\\') === 0
    && ($file = $classLoader->findFile($class))
) {
    \Composer\Autoload\includeFile($file);
    class_alias($class, 'Zend\\' . $class, false);
    return;
}

// next condition here

I'd also import the Composer\Autoload\includeFile function.

[michalbundyra]

@weierophinney

Combine the statements, and return early

Actually it's not gonna help, because now we have elseif, and if we combine ifs then condition from elseif is going to be checked in case strpos($class, 'Expressive\') is satisfied but the file is not found.

This whole part is going to be changed anyway - as we need to have reverse mapping, then we will do the same as in append preloader - split class name into segments and try to find quicker the legacy name.

I'd also import the Composer\Autoload\includeFile function.

Unfortunately, we cannot. As importing function is available form PHP 5.6. We've decided to support PHP 5.3 here.

[michalbundyra]

@weierophinney This PR is now ready to go!