Ensure compatibility with frozen string literals (#643)

Ref: https://bugs.ruby-lang.org/issues/20205

Ruby is moving forward with enabling frozen string literals
in the future.

Reline being part of the ruby-core test suite should work when
ruby is ran with `--enable-frozen-string-literal`.

Co-authored-by: Jean Boussier <jean.boussier@gmail.com>

.github/workflows/reline.yml

@@ -35,6 +35,10 @@ jobs:
         env:
           TERM: xterm-256color
         run:  bundle exec rake test
+      - name: rake test (frozen string literal)
+        env:
+          TERM: xterm-256color
+        run: bundle exec rake test RUBYOPT="--enable-frozen-string-literal"
 
   readline:
     name: >-

lib/reline/kill_ring.rb

@@ -68,7 +68,7 @@ def initialize(max = 1024)
   def append(string, before_p = false)
     case @state
     when State::FRESH, State::YANK
-      @ring << RingPoint.new(string)
+      @ring << RingPoint.new(+string)
       @state = State::CONTINUED
     when State::CONTINUED, State::PROCESSED
       if before_p

lib/reline/line_editor.rb

@@ -1256,7 +1256,7 @@ def editing_mode
         item_mbchars = item.grapheme_clusters
       end
       size = [memo_mbchars.size, item_mbchars.size].min
-      result = ''
+      result = +''
       size.times do |i|
         if @config.completion_ignore_case
           if memo_mbchars[i].casecmp?(item_mbchars[i])
@@ -2935,7 +2935,7 @@ def finish
   end
 
   private def ed_delete_prev_char(key, arg: 1)
-    deleted = ''
+    deleted = +''
     arg.times do
       if @cursor > 0
         byte_size = Reline::Unicode.get_prev_mbchar_size(@line, @byte_pointer)

lib/reline/windows.rb

@@ -85,7 +85,7 @@ def initialize(dllname, func, import, export = "0", calltype = :stdcall)
       def call(*args)
         import = @proto.split("")
         args.each_with_index do |x, i|
-          args[i], = [x == 0 ? nil : x].pack("p").unpack(POINTER_TYPE) if import[i] == "S"
+          args[i], = [x == 0 ? nil : +x].pack("p").unpack(POINTER_TYPE) if import[i] == "S"
           args[i], = [x].pack("I").unpack("i") if import[i] == "I"
         end
         ret, = @func.call(*args)