Unaligned write of u64 on 32-bit and 16-bit platforms

Affected versions allocate memory using the alignment of usize and write data
to it of type u64, without using core::ptr::write_unaligned. In platforms
with sub-64bit alignment for usize (including wasm32 and x86) these writes
are insufficiently aligned some of the time.

If using an ordinary optimized standard library, the bug exhibits Undefined
Behavior so may or may not behave in any sensible way, depending on
optimization settings and hardware and other things. If using a Rust standard
library built with debug assertions enabled, the bug manifests deterministically
in a crash (non-unwinding panic) saying “ptr::write requires that the pointer
argument is aligned and non-null”.

No 64-bit platform is impacted by the bug.

The flaw was corrected by allocating with adequately high alignment on all
platforms.