common: replace BitVector::NoInitAllocator with wrapper struct

common: replace BitVector::NoInitAllocator with wrapper struct

in c++20, the deprecated `struct std::allocator<T>::rebind` template was
removed, so `BitVector` no longer compiles. without a `rebind` to
inherit, `std::allocator_traits<NoInitAllocator>::rebind_alloc<U>` was
looking for `NoInitAllocator<U>`, but it isn't a template class

further investigation found that in c++17, `vector<__u32, NoInitAllocator>`
was rebinding this `NoInitAllocator` to `std::allocator<__u32>` and
preventing the no-init optimization from taking effect

instead of messing with the allocator to avoid zero-initialization, wrap
each __u32 in a struct whose constructor does not initialize the value

Fixes: https://tracker.ceph.com/issues/54279
Signed-off-by: Casey Bodley <cbodley@redhat.com>