Add an alias (UInt64) to uint64_t.
Divide types.hpp into several headers.
Include a header for Datum from types.hpp.
Include headers for Array<T> from types.hpp.
Include a header for Error from types.hpp.
Add a missing entry (tree_index.hpp) to Makefile.am.
Enable ref_table of ColumnImpl<Vector<Int>>. (#48)
Add tests for Vector<Int> reference column. (#48)
Support subexpressions for Vector<Int>. (#48)
Add tests for Vector<Int>. (#48)