[SuffixTree] Add suffix tree statistics

Sometimes you want to see how much is being allocated in your data structure
in general.

Add statistics that show how many internal and leaf nodes have been allocated
in the suffix tree over the course of its construction.

Also add a testcase that shows that we actually get these stats out when we're
outlining stuff.

The test shows that we get the expected O(n) leaf nodes, a split, and so on.