Safe optimizations

[wingo]

Change Chez, Gambit, and Gerbil to avoid unsafe optimizations. Fixes #43.

[ghost]

If we're heading that way, then you might also want to change https://github.com/ecraven/r7rs-benchmarks/blob/master/bench#L760.

${GERBIL} -d $(dirname "$1") -exe -static -O -prelude "(declare (not safe))" -o "${1%.scm}.exe" "$1"

[belmarca]

Why not just have a safe and unsafe benchmarks?

Maybe a "defaults" benchmark?

[svenha]

Producing benchmark results in safe mode and in unsafe mode will help a lot of Schemers.

A defaults run could help beginners, but beginners should not benchmark too much :-) So, I would avoid a defaults run.