Add $_GET wrapper that allows caches to analyze usage

Issue Summary

Currently, my cache forces the cached function and caller to determine in what ways the cached function depends on $_GET and restore necessary state manually. Adding and using this wrapper would allow a cache to determine -- based on before and after snapshots of usage -- what $_GET variables were used in any function's execution. With that knowledge, the cache could selectively save and restore these variables during cron runs to regenerate the content.