tbl: reimplement _Row.cells

Temporarily named _Row.cells_new
tbl: add _Row.table
tbl: add _Rows.table
tbl: add Table.table
tbl: add _Row._index
tbl: add Table.row_cells()
tbl: add Table._column_count
test: add snippet infrastructure
tbl: add Table._cells
tbl: remove now-dead code

* remove _RowCells and tests
* make _Row.cells_new _Row.cells and remove prior version
acpt: fix paste-repeated typo in several features
tbl: reimplement _Column.cells
tbl: add _Column.table
tbl: add _Columns.table
tbl: add _Column._index
tbl: add Table.column_cells()
tbl: remove now-dead code
tbl: refactor Table.cell()
acpt: add scenarios for _Cell.merge()
tbl: reorder methods in Describe_Cell

No code changes, just changing ordering of test methods.