Change how we store the scope chain --- instead of always using a separate ScopeChainNode, we store a single link inline inside ActivationImp's when possible.

This gives a (disappointing) 3.1% speedup on SunSpider, but on the plus side:
1. Control-flow-intensive benchmarks are as much as 10-20% faster
2. Improvements are pretty much accross the board.
Plus, this should mesh nicely with multi-level static binding lookup improvements.