alloc-no-stdlib 3.0.0 moved its C calloc backing store behind the "unsafe"
feature.

We only utilize StackAllocator/Allocator/SliceWrapper traits,
so we are compatible with both versions of alloc-no-stdlib and alloc-stdlib

For seccomp, we need CallocBackingStore via
define_allocator_memory_pool!(.., calloc), so this requires
alloc-no-stdlib/unsafe

Tested against both 2.0.4 and 3.0.0.