Premature calculation of actual description when generating matcher description

[jas14]

In cases where the example passes, SuperDiff::RSpec::AugmentedMatcher#description appears to generate the description of the actual object via a call to matcher_text_builder_args:

https://github.com/mcmire/super_diff/blob/805537f7b138a82ab45a64e09cac332bf21816c7/lib/super_diff/rspec/augmented_matcher.rb#L43

However, the MatcherTextBuilders::Base#matcher_description doesn't use said description:

https://github.com/mcmire/super_diff/blob/805537f7b138a82ab45a64e09cac332bf21816c7/lib/super_diff/rspec/matcher_text_builders/base.rb#L30-L39

This can incur a substantial delay in cases where the inspection of the actual object takes a lot of time (e.g. #160).