This data structure is basically a vector, items of which require no
initialization. An item is initialized to default value the first time
it's accessed. Clearing of all values takes O(1) time.

It's handy for use cases when you have to frequently perform such
operations on a big array:
1) initialize a few items in it
2) clear all items

Inspired by this article: http://bit.ly/bDHfzA