On some systems, uint_fast8_t may be as big as size_t! So the `static_assert(sizeof(aIndex) < sizeof(size_t))` could fail there. The better test here is to check for the expected type (uint_fast8_t).
Now, since uint_fast8_t can be bigger than 8 bits, we may as well choose it for variant sizes greater than 255, up to UINT_FAST8_MAX.
(The added parentheses help clang-format distinguish '<' for tests vs for templates.)
Differential Revision: https://phabricator.services.mozilla.com/D119574
