Define internal accessors on module state structures

[xandkar]

As per @vihu's original suggestion at https://github.com/helium/erlang-hbbft/pull/62#discussion_r585546569

We don't need to go too-crazy, since theses structures are not accessed from outside the module, hence we could score wins only in cases of common combinations, such as B/N (which first needs to extract both B and N from state record). Perhaps also in cases where pattern matching gets a little too noisy.