Add a MultiInstanceFile to allow many file handles to the same resource

[bizurkur]

This should allow any number of handles to be opened to the same vfs resource, but each maintain separate pointers to the place in the file they are.

• Handle A should know it's at position X
• Handle B should know it's at position Y
• Each file handle should not interfere with the other file handle's position when performing operations

In other words, this is now possible (how it works with normal files):

$root = vfsStream::setup();
$file = vfsStream::newFile('foo')->setContent('baz2')->at($root);

$fp1 = fopen($file, 'rb');
$fp2 = fopen($file, 'rb');

$this->assertEquals('baz2', fread($fp1, 4096));

// previously this would be a blank string, as fp1 read all the data
$this->assertEquals('baz2', fread($fp2, 4096)); 

fclose($fp1);
fclose($fp2);

Closes #129

[mikey179]

Overall I'm not against this, as this solves a bug in the library and doesn't introduce anything that clutters the public API. My only gripe with it right now is the name MultiInstanceFile.

Having said that, for me it's another piece in the puzzle of how things could be restructured internally. I can't put them all together in my mind quite yet, but my idea for improving the class names in #213 is part of that. Especially the implementation of restorePosition() and savePosition() show that there's massive room for improvement and that maybe not all informations are at the place where they ought to be.