rename bits to bitstring?

[StefanKarpinski]

Mentioned here, the bits function, as named, should return a vector of Bools, but instead returns a string of '0' and '1' characters that are a convenient representation of the bits of the item in question. Rather than change the functionality to something less convenient, I propose renaming the function to bitstring which indicates that it returns a string representing the bits.

[JeffBezanson]

Closed by #24281.