- User-registered kernels are contained in the `ukr_t`, `bszid_t`, and `ukr_pref_t` enums, so prefer `siz_t` for all functions taking kernel ID parameters.
- However, because enum constants are by default `int` (or even smaller), while `siz_t` will typically be `long long unsigned int`, this causes problems with variadic arguments in `bli_cntx_set_ukrs` and friends (predefined vs. user-registered kernel IDs will be incompatible types).
- Solution: define `BLIS_VA_END` explicitly as `(siz_t)-1` and add an enum entry equal to `BLIS_VA_END` (=`0xf...f`) to each affected enum in order to force the underlying enum type to be as wide as `siz_t`.

Return a bool which is true iff the operation was transposed (C = A B -> C^T = B^T A^T).