REF: pytables do string conversion early to set attributes in fewer places
Next-step following #29979.

A lot of work is currently done in set_atom_string (a self-mutating method) that can be done in an earlier, not-mutating call.  By making that move, we simplify set_atom quite a bit, and get closer to getting all the attribute-setting into the constructor
thanks, that does simplify the nested logic a bit i figured out last night it allows further simplfications even more than i expected.  coming up before long.