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)

(wen)