Avoid forming misaligned typed pointers in (de)serialization

[lemire]

Summary

Two places formed misaligned typed pointers that were only ever passed to memcpy:

• roaring64_bitmap_frozen_serialize accepts an unaligned output buffer (only roaring64_bitmap_frozen_view requires 64-byte alignment), but it carved the output into uint64_t * / uint16_t * / rle16_t * cursors aligned relative to the buffer start. With an unaligned base pointer (as check_frozen_serialization in roaring64_unit deliberately uses), those typed pointers are misaligned.
• roaring_bitmap_deserialize and roaring_bitmap_deserialize_safe formed const uint32_t *elems at buf + 5 and computed elems + i before the memcpy. The comment already said the data may be unaligned; the pointer type just didn't match that intent.

Nothing faults at run time since the copies go through memcpy, but forming a misaligned pointer to a type is undefined behavior (C11 6.3.2.3p7). clang's -fsanitize=alignment checks typed memcpy arguments and reports both; GCC's UBSan (what ubuntu-sani-ub-ci uses) does not, which is why CI is green.

The fix keeps byte (char *) cursors and advances them by byte counts. container_get_frozen_size() already returns the byte count copied, so container_frozen_serialize now advances by it directly. Serialized output is byte-for-byte unchanged.

Test plan

• clang + -DROARING_SANITIZE=ON -DROARING_SANITIZE_UNDEFINED=ON: 27/27 tests pass, zero runtime error lines in the ctest log (previously roaring64_unit reported a misaligned uint16_t * store in container_frozen_serialize, and toplevel_unit aborted in test_serialize at roaring.c:1687)
• Release build: 27/27 tests pass
• clang-format 17 clean

[Dr-Emann]

It always seems crazy that just creating an unaligned pointer is UB in C, something that always catches me when going from rust to C/C++.

[lemire]

@Dr-Emann I don't think it actually matters. I am being pedantic here. I would not fix it if not for the sanitizer warnings.