Removing recursion from find_mut in treemap

Removing recursion from TreeMap implementation, because we don't have TCO. No need to add ```O(logn)``` extra stack frames to search in a tree.

I find it curious that ```find_mut``` and ```find``` basically duplicated the same logic, but in different ways (iterative vs recursive), possibly to maneuvre around mutability rules, but that's a more fundamental issue to deal with elsewhere.

Thanks to acrichto for the magic trick to appease borrowck (another issue to deal with elsewhere).