Optimize dynamic sequence lazy dependencies

Avoid a couple extra object allocations and simplify memory management.
Only autorelease RACDynamicSequence tails sometimes
Add missing import
Pull sequence binding dependencies out of the block
Add a private class specifically for unary sequences
Optimize parts of empty and unary sequence binding
Enumerate eager sequence binding results with fast enumeration
Optimize fast enumeration for RACArraySequences
Skip the objectEnumerator in left folds

It's not going to make any difference when `self` is retained in the current scope.
Merge pull request #461 from ReactiveCocoa/sequence-optimization

Horrible things to optimize the hell out of sequences