Improve Our Handling of Subnodes

[Adlai-Holler]

• Remove ASDisplayNode's conformance to NSFastEnumeration
  • We were creating a new copy of subnodes for each chunk of enumeration.
  • The enumerated collection wasn't stable – technically nodes could be deallocated during enumeration under the right circumstances.
• Instead of making a new copy of subnodes each time, just store a cached copy.
• Remove silly dealloc line that explicitly nils the ivar.

[Adlai-Holler]

This is a breaking API change (removing the conformance) but I think the impact will be small because most people probably just iterate through node.subnodes.

[garrettmoon]

This seems like a dangerous optimization. Could we at least put in an assert that _subnodes and _cachedSubnodes have the same content in the subnodes getter?