Moved macros for handling specific key or value types to separate

include files. This cleaned up the code quite a bit and should make it
pretty easy to add additional types in the future.

Fixed numerous errors with handling bucket list pointers, deletions of
subtrees, etc.

Fixed bugs in items indexing.

Moved macros for handling specific key or value types to separate
include files. This cleaned up the code quite a bit and should make it
pretty easy to add additional types in the future.

Changed BTreeItems_length so it lies. Waaa. See comment in method.