Structure recreate support

[michael-donat]

@milesj in #15 mentioned following:

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On a side note, it would be nice to have a method like createStructure() that can easily create nested files and folders.

$vfs->createStructure([
    'folder1' => [
        'file1.txt' => 'contents',
        'file2.php' => '<?php',
        'folder2' => []
    ]
]);

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[milesj]

👍