XML Processor: Add auto pulling semantics

[adamziel]

Add support for Byte Streams in XMLProcessor to move through larger documents without explicit is_paused_at_incomplete_input() calls:

$processor = XMLProcessor::create_for_streaming(
	FileReadStream::from_path(__DIR__ . '/fixtures/10MB.xml')
);
$processor->next_tag('content:encoded');

// A new handy utility to collect all cdata and text nodes contents:
$text = $processor->get_all_modifiable_text_until_next_tag();
// The entire text will be stream-loaded from the disk – even if there's 40MB of it