`exp_short_init_compact_seq` uses both SEXP and void as return types (should just be void)

[DavisVaughan]

Switch to using void consistently:
https://github.com/search?q=repo%3Ar-lib%2Fvctrs%20exp_short_init_compact_seq&type=code

Unfortunately we copied the SEXP definition into slider when we imported it so we will need to patch slider:
https://github.com/r-lib/slider/blob/af02bdaccfc332604f55908d9cc35d63cb1d5c7a/src/slider-vctrs-private.c#L18

This caused a clang-san issue when CRAN ran incoming checks for clock. It's not a problem for clock but is a problem for vctrs/slider.

Flavor: r-devel-linux-x86_64-debian-special-clang-san
Check: Post-processing issues found for clang-san, Result: WARNING
  File: tests/testthat.Rout
  index.c:444:3: runtime error: call to function exp_short_init_compact_seq through pointer to incorrect function type 'struct SEXPREC *(*)(int *, int, int, bool)'

I think I can do the slider fix first, which should immediately fix the asan runtime error, then align the vctrs problem (and we may not even need an immediate release for that)

[DavisVaughan]

Fixed in #2103