Add a method to iterate through a MetadataSourceIter

[tomjnixon]

In general we can't iterate through all MetadataSource implementations, because get_next_block is allowed to return None if metadata is not yet available, but might become available in time for rendering.

Allowing iteration through a MetadataSourceIter would be fine though, and would make using rendering items in scripts where we know we're always going to have a MetadataSourceIter a bit nicer.