Add CI on Github actions with RSpec tests in addition to improvement of Rake tasks and loading

[fenrir-naru]

Modernization of this gem has been performed. This PR icludes the following features.

• The main modification point is to support continuous integration on Github actions. The RSpec tests will be performed on Ruby 2.3 to 4.0 in ubuntu and windows platforms like this.
• Popular development style with bundler and rake, in which bundle install to prepare for developement, bundle exec rake to compile and test this gem, and bundle exec rake yard to generate document, are introduced.
• The loading rule forcing a user manually to require 'numo/narray' before require 'numo/fftw' is relaxed. Only require 'numo/fftw' is enough now.