An implementation of B trees. The core operations on B trees
(lookup, iterate, put and del) are generic in the actual
implementation of nodes, via the BTreePage and
BTreeMutPage. This allows for a simpler code for the
high-level functions, as well as specialised, high-performance
implementations for the nodes.
Types that can be stored on disk. This trait may be used in
conjunction with Sized in order to determine the on-disk size,
or with UnsizedStorable when special arrangements are needed.