Value History tracker

[hgzimmerman]

Add a struct that stores values in a Vec where the tail element of the vec represents the most recent change.

Notable methods could include:

• forget - drop all elements except the last element.
• reset - drop all elements except the first element.
• dirty - returns true if the len of the vec > 1. (maybe a specific value could be used to mark this)
• make_older - deletes the current last element (???)

Maybe a VecDeque might be a better underlying data structure. That would allow the first element to efficiently represent the "current" value.