Previously this class had a large number of instance variables being
used as storage, however they were almost entirely being accessed in a
hash-like way using send and instance_variable_set, both of which are
slower than just using a hash.

By just using a hash we make this simpler and faster. We can also keep
the hash sparse rather than using `nil` as a "not set" sentinel value.

These were only used in one place. We can avoid complexity by not
defining them.

For some reason Rubocop doesn't like the case equality operator (it's
good) so we can just use a case statement I guess.