Sludge crawler and field improvements

Part one of the changes needed to properly implement the sludge crawler.

This results in a generally not-so tough creature with a cool trail effect.

The discovery process that has revealed that the first priority for implementing the sludge crawler will be fixing fields, and part one gets that started, putting in a basic framework for slowing critters based on fields and doing quite a bit of cleanup of the field code in preparation for next steps.

Current status is stable, though nothing as is makes use of the field slowing effects.