Work in progress on the B tree.

Values are now all stored in the leaves. Erase does not work at the
moment, and it is assumed that sizeof(Key) == sizeof(Value).

In a single run on a release build, the ut-blocks b_tree_2 test with
n = 8388608 succeeded in 0m51.350s (real; 0m16.269s user; 0m31.750s sys).