Lab 5: find() on non-existent entries

[aaronbloomfield]

Moving from aaronbloomfield#186:

We should specify whether find() may be called to look for a string that isn't in the tree. If it can, then we need to specify what to do with "LeftLinksFollowed" and "RightLinksFollowed" (i.e. do we count one more link to reach the "null" terminator, or no?)