Add ruby 3.x support for separately-tracked kwargs

[apiology]

Teach cache_method to track kwargs separately from args, rather than treating them as a hash arg in the final position.

Fixes #17

[apiology]

I dropped this in my Gemfile to be use this fork until someone is able to merge and release a new gem.

# Awaiting merge and release of
# https://github.com/seamusabshere/cache_method/pull/18
gem 'cache_method',
    git: 'https://github.com/apiology/cache_method',
    branch: 'ruby_3_support'