Fix some build errors.
Add the files for BTree and BTreeIndexPage.
Rename DataPage to BTreeDataPage.
Switch to a sorted implementation of BTreeDataPage.
Make some minor improvements to BTreeDataPage.
Make it possible for tuples and fields to be read-only - it is important not to modify them when they are in a sorted data page.
Start working on BTree.
Implement PrefixTupleComparator.
Rework BTreeDataPage to use a prefix tuple comparator.
Delete tuples_by_range() and tuples_by_value() from BTreeDataPage - the functionality they provide is no longer needed.
Improve the interface to BTreeDataPage.
Start working on BTree::Iterator.
Make it possible to copy and assign BTreeDataPage.
Fix a compilation error.