Storage improvements

targets 1.2.1

• Add tar_prune_list() (#1090, @mglev1n).
• Wrap file.rename() in tryCatch() and fall back on a copy-then-remove workaround (@jds485, #1102, #1103).
• Stage temporary cloud upload/download files in tools::R_user_dir(package = "targets", which = "cache") instead of tempdir(). tar_destroy(destroy = "cloud") and tar_destroy(destroy = "all") remove any leftover files from failed uploads/downloads (@jds485, #1102, #1103).
• Use paws.storage instead of all of paws.