Specialize Vec::from_elem to use calloc
Fixes#38723. This specializes the implementation for `u8` only, but it could be extended to other zeroable types if desired.
I haven't tested this extensively, but I did verify that it gives the expected performance boost for large `vec![0; n]` allocations with both alloc_system and jemalloc, on Linux. (I have not tested or even built the Windows code.)
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