Allow `@cache` to ignore certain arguments

[AayushSabharwal]

A function may mark an argument to be ignored by @cache. The function is then responsible for making sure the value of that argument doesn't affect the validity of the cached result. This is useful for substitute, where we don't want to cache the dict of substitution rules. Instead, substitute uses a cached worker function and clears the cache every call. We could also have a flag if the user knows they're calling substitute with the same ruleset multiple times, to avoid clearing the cache.