API/PERF: add a performance string-object memory computation

[jreback]

xref #8599, #8578

allow a nice UX and a more correct estimate of memory usage for object (string) columns
prob will require a cythonized evaluation

this would replace the need for the '+' on object dtypes for mem usage

[mroeschke]

I think my moving to pyarrow for string ops this becomes irrelevant. Closing but happy to reopen if i misunderstodd