Temporary solution to weak strong dance

[clayellis]

This has already been discussed at length:

• Easy strongified weak `self` in closures - #15 by Jon_Shier
• [Proposal] Allow upgrading weak self to strong self by assignment - #9 by Shawn_Erickson
• Proposal: weakStrong self in completion handler closures
• Guarded closures - #10 by acecilia
• [Draft] Guarded closures and `@guarded` arguments
• $self