Problem:
- Clang ToT generates warnings due to user-declared functions causing
the implicitly generated assignment operator to not be generated.
Solution:
- Declare the default constructor `= default`.
- Remove the default copy constructor declaration.
Unfortunately this patch is quite large.
The main functionality included are a BTree index implementation and
the Heap class which manages persistent storage.
Also included are a Key subclass of the Tuple class, which is a
specialization for index key tuples. This "dragged in" the Meta layer,
which has classes defining SQL objects like tables and indexes.