Add `frozen_string_literal: true`

[ooooooo-q]

I used memory_profiler to check where rubyXL was using a lot of memory, and fixed it.

Added frozen_string_literal: true to lib/ files. Testing seems to be fine, but depending on how users use the library, it may have an impact.

Result

• write xlsx
  • https://github.com/ooooooo-q/rubyxl_memory_usage/blob/main/operate.rb#L8
• read xlsx
  • https://github.com/ooooooo-q/rubyxl_memory_usage/blob/main/operate.rb#L42

rubyXL 3.4.20 (with Ruby 3.1.0)

• write file
  • Total allocated: 16.41 MB (335760 objects)
  • Total retained: 6.58 kB (128 objects)
• read file
  • Total allocated: 24.26 MB (340069 objects)
  • Total retained: 6.23 kB (57 objects)

frozen_string_literal

• write file
  • Total allocated: 15.56 MB (314304 objects)
  • Total retained: 6.58 kB (128 objects)
• read file
  • Total allocated: 24.02 MB (334013 objects)
  • Total retained: 6.23 kB (57 objects)

[weshatheleopard]

I do not believe these savings are significant. Besides, recent ruby automatically freezes literals (which caused issue fixed by 076ccdae048423bcb1bf31a459c5a1374378c20e)

[ooooooo-q]

I seem to remember that automatically freezes literals in ruby3 has been abandoned, but I may be mistaken. https://bugs.ruby-lang.org/issues/11473#note-53