acpt: add scenarios for Table collections length
tbl: change _RowCells.__len__() algorithm

This change introduces the _table property to propagate access to the
parent table. A couple feature tests need to be returned to WIP while
that call chain is implemented.
tbl: add _Row._table
tbl: add Table._table
tbl: add _Rows._table
acpt: fix paste-repeated typo in several features
spike: passes Access cell in row
spike: passes Access cell sequence of column scenario
spike: passes Access cell by row and column index
spike: refactor tbl-cell-access.feature

Remove now redundant len() scenarios
spike: return tuple from _Row and _Column.cells
spike: spike Cell.merge()