Refactor by creating Term, Amount and ExciseTaxRate classes

[mi-wada]

Why

Increase maintainability by eliminating duplicate logic and reducing complexity

What

Refactored by creating Term, Amount and ExciseTaxRate classes.
It provides the following advantages:

• Made duplicate logic DRY
• Easy to understand

I think the correctness of this change can be guaranteed since it does not change existing test code like test/jct/excise_test.rb.

[mi-wada]

Until now, the number of days has been calculated using (start_on..end_on).count. However, from the following simple benchmark, this method was faster than (start_on..end_on).count, so I changed it.

# $ ruby -v
# ruby 2.7.2p137 (2020-10-01 revision 5445e04352) [arm64-darwin21]

require 'benchmark'
require 'date'

n = 10000

d20200101 = Date.new(2020, 01, 01)
d20210101 = Date.new(2021, 01, 01)

use_count = Benchmark.realtime do
  n.times do |_|
    _count = (d20200101..d20210101).count
  end
end

unuse_count = Benchmark.realtime do
  n.times do |_|
    _count = (d20200101 - d20210101 + 1).to_i
  end
end

puts "use_count execution time: #{use_count}"
puts "unuse_count execution time: #{unuse_count}"

# => use_count execution time: 0.39297799998894334
# => unuse_count execution time: 0.0031369999051094055

[mi-wada]

for reviewers

Sorry for the large difference.
If splitting the PR makes it easier to review, I will do so, so please give me your opinion.