Log Message:
Update to 1.60

Upstream changes:
1.60 -- 2021-10-08
        [BUGFIXES]
         * Quiet some -Wsign-compare warnings by using `int` instead of `UV`
           (RT136985)
         * Quiet a -Wmaybe-uninitialized warning by ensuring a variable is
           always initialised, because the compiler can't always determine this
           is safe (RT139356)
         * Avoid SIGFPE in product(-1, ...) (RT139601)

1.59 -- 2021-09-12
        [CHANGES]
         * Removed Scalar::Util::isbool() as the API design for older perls
           isn't quite clear, and it doesn't do anything useful on any
           currently-released perl version
          * Tidy up the unnecessary bits of @EXPORT_FAIL
          * Use the new G_LIST constant rather than G_ARRAY

1.58 -- 2021-09-12
        (bad release because I forgot to `git push`)

1.57 -- 2021-09-11
        [CHANGES]
         * Added Scalar::Util::isbool() to track new core-supported boolean
           value nature on scalars
         * Fix various typoes in docs, code comments, etc...
         * Removed code conditional on whether SvWEAKREF is supported, as
           every perl since v5.6 supports it

        [BUGFIXES]
         * Fix segfaults when an undefined subroutine is passed to
           block-taking subs (thanks ilmari)
         * Correct refcount handling on hv_store_ent to PL_DBsub in
           set_subname() (thanks nwc10)
         * Don't rely on Scalar::Util functions when unit-testing List::Util
           ones (thanks haarg)