Weak Block

[alexpersian]

Motive: The Block type alias is very useful, but it means it's not always clear that self will be strongly captured when using.

Proposal: Add a second type alias called WeakBlock that expects a [weak self] capture list to ensure that strong reference cycles don't occur.

• Alternate solution: document the Block type alias to indicate that references should be weak.

Edit: so capture lists can't be specified through type aliases, so I'm going to solve this through documentation.

[alexpersian]

Addressed through #137