util.indexedbheap: A priority queue implementation with a reverse index with no per-entry memory allocation.
util.timer: Updated to use util.indexedbheap to provide a more complete API. Timers can now be stopped or rescheduled. Callbacks are now pcall'd. Adding/removing timers from within timer callbacks works better. Optional parameter can be passed when creating timer which gets passed to callback, eliminating the need for closures in various timer uses. Timers are now much more lightweight.
util.timer: Fix variable name typo.
util.timer: Fix another variable name typo (thanks again zash).
util.timer: Import all require upvalues.