Ensure that the OP_Prev opcode verifies that content has not been deleted
out from under the cursor.  Fix for ticket [209d31e3161b9e9ff].
Fix the header comment in sqlite3VdbeDeletePriorOpcode().  No changes to
code.
Remove the SQLITE_ENABLE_TREE_EXPLAIN compile-time option.  Add alternative
debugging display routines: sqlite3TreeViewExpr(), sqlite3TreeViewExprList(),
and sqlite3TreeViewSelect().
Show tree diagrams of data structures in the debugging output when the 0x100
bit is set on sqlite3WhereTrace or sqlite3SelectTrace.
Enhanced debug output for OR-logic in the query loop optimizer.
Further enhancements to the "wheretrace" debugging output.
Enable the query planner to deal with WHERE clauses that have OR terms
nested within AND terms that are nested within OR terms.  Also remove an
unused function declaration.
Improvements to the new syntax-tree output routines:  Omit the "END SELECT"
mark and instead terminate the graph at the last item.  Increase the maximum
tree depth to 100.
Merge the latest enhancements from trunk.