Implement reset for memoized

Created by: ka8725

It would be nice to have a method to reset the memoized store. The store resetting would allow to run example several times in the around block. That in turn, could allow to define options on contexts that specify when the example should be run multiple times in different environment.

Specifically, the next option on contexts can be defined:

config.around(:each, within_time_zones: ->(value) { value.present? }) do |example|
  example.metadata[:within_time_zones].each do |zone|
    __memoized.reset
    Time.use_zone(zone) { example.run }
  end
end

With it help it would be easy to run context in different timezones:

context 'some test', within_time_zones: ['Madrid', 'Minsk'] do
   ...
end

Unfortunately, without a reset method there is the only way to implement that option for context using instance_variable_set that's not very elegant: __memoized.instance_variable_set(:@memoized, {}).