Add RBS files and tests

Gemfile

@@ -6,4 +6,9 @@ group :development do
   gem "rake"
   gem "test-unit"
   gem "test-unit-ruby-core"
+
+  # RBS requires Ruby >= 3.0
+  if RUBY_VERSION >= "3.0.0"
+    gem "rbs", "~> 3.4.0.dev", require: false, git: "https://github.com/ruby/rbs.git", branch: "master"
+  end
 end

sig/integer-extension.rbs

@@ -0,0 +1,41 @@
+%a{annotate:rdoc:skip}
+class Integer
+  # <!--
+  #   rdoc-file=lib/prime.rb
+  #   - each_prime(ubound) { |prime| ... }
+  # -->
+  # Iterates the given block over all prime numbers.
+  #
+  # See `Prime`#each for more details.
+  #
+  def self.each_prime: (Integer) { (Integer) -> void } -> void
+
+  # <!--
+  #   rdoc-file=lib/prime.rb
+  #   - from_prime_division(pd)
+  # -->
+  # Re-composes a prime factorization and returns the product.
+  #
+  # See Prime#int_from_prime_division for more details.
+  #
+  def self.from_prime_division: (Array[[ String ]]) -> Integer
+
+  # <!--
+  #   rdoc-file=lib/prime.rb
+  #   - prime_division(generator = Prime::Generator23.new)
+  # -->
+  # Returns the factorization of `self`.
+  #
+  # See Prime#prime_division for more details.
+  #
+  def prime_division: (?Prime::PseudoPrimeGenerator) -> Array[[ Integer, Integer ]]
+
+  # <!--
+  #   rdoc-file=lib/prime.rb
+  #   - prime?()
+  # -->
+  # Returns true if `self` is a prime number, else returns false. Not recommended
+  # for very big integers (> 10**23).
+  #
+  def prime?: () -> bool
+end

sig/manifest.yaml

@@ -0,0 +1,2 @@
+dependencies:
+  - name: singleton