Make a version of `dump_bits` that returns a string instead of dumping to stdout.

[bmhowe23]

We already have dump_bits in the tensor class, so I'd rather use that one. It currently returns void and dumps the values to stdout, but I'd be willing to change that and just have it return a std::string (which is kind of what this mat_to_str function is doing). In the interest of isolating changes, I'd rather do that in a new PR. So I'll convert this one to a new GitHub issue before merging this PR.

Originally posted by @bmhowe23 in #147 (comment)