BTreeG: consider using a generic type for the key

[RaduBerinde]

In many cases the T type used with BTreeG contains the key and other fields (in some cases, many other fields). It is awkward to create a "full" T object with only the key initialized just to pass to Get() or Delete() or AscendRange().

My proposal is to define K as the key type and create the btree using a LessFunc<K> as well as a func (t T) K which returns the key from a T object. Methods like Get(), Delete(), or AscendRange() would only take a K. The current BTreeG can be reimplemented in terms of this tree, with K = T.