AgentIterator

This exists purely as a performance hack. Normal code can just call AgentSet.agents which returns an Iterable[Agent]. But in performance-critical code, using AgentIterator has a performance advantage because type erasure means that if we use Iterator[Agent] there are typecasts going on under the hood, and I found that this actually impacts performance on benchmarks (to the tune of 5% or so on Life Benchmark, for example). Sigh... - ST 2/9/09