tor-bytes: Migrate to fallible writers

We should migrate to fallible implementations of Writeable::write_onto, WriteableOnce::write_into, and the functions that call them. We should do this in a piecemeal way, rather than rewriting everything at once.

One strategy would be to add new fallible versions of the functions to the traits, with default implementations. Then we can modify all the callers, one by one, to either use (or have a variant that uses) the new traits. Then we can migrate Writeable implementations as needed.