Unify formatting floats and f{32,64}::to_str_digits

[erickt]

cc @alexcrichton. It looks like we have a lot of duplicated code with libcore/fmt/float.rs and std::num::strconv::float_to_str_bytes_common. Can this code be merged?

[alexcrichton]

I'd love to remove the libstd version! I think that it did a thing or two that the libcore version didn't do (hence the duplication), but those bits could probably just be removed.

[steveklabnik]

I'm not sure how this has gottne shuffled, but to_str_digits nor most of these modules still exist. Let me know if this refactor is still worthwhile!