Class Make.cache

new cache finder ?timer size creates a cache object where finder is the function responsible for retrieving non-cached data, timer (if any) is the life span of cached values (in seconds) (values in the cache are removed after their time is up) and size is the upper bound to the number of simultaneoulsy cached elements.

Whenever a value is found (using find method), it's lifespan is set to timer (or not if the cache is not time bounded). If the value was already cached, it's lifespan is reset to timer.

Using timer allow one to create a cache bounded both in space and time. It is to be noted that real lifespan of values is always slightly greater than timer.