Rehash

[mdsumner]

New approach post tidync:

• make all indexing lazy
• print out tbl_cube equivalent + skimr
• skimr of dims and valid-ranges
• require explicit collect()

[mdsumner]

lazy notes for self:

• tibble::trunc_mat relies on a head.tbl_lazy method, and sets n in the op_single
• format.tbl takes that and runs cat_line() on it to produce the lazy tbl db print