Ensure Data.List functions are nondestructive

[tyru]

via #511 (comment) (thanks @Milly)

• Remove unnecessary copy() from test files
• Add tests to check if functions are nondestructive
• Remove unnecessary copy() from document
• Note all functions are nondestructive in the document
  • Data.List.uniq(), uniq_by() are noted already
  • Is it enough to note all functions are nondestructive in introduction section?

[ujihisa]

Since existing Data.List functions are not necessarily nondestructive, I'll clarify destructive/nondestructive for each functions in the documentation, as the first step.

[ujihisa]

According to the comment we aren't changing Data.List entirely to non-destructive. Since all the functions with {list} argument clearly mention if they are destructive or non-destructive, I close this now.