Performance Improvements

This series increases performance of (fib 25) by about 25% total. Some commits help more than others, but at least on my system, they all improve the benchmark somewhat. They all seem correct, too, so that helps.

The biggest one is the fix to Env.Find()

To find these, I've been using (needs the benchmark from the beginning of the series):

    go test -test.bench=. -cpuprofile=cpu.prof

To generate a profile, and

    go tool pprof ./golftalk.test cpu.prof
To examine the profile.