Miri reported this here:

Undefined Behavior: memory access failed: alloc42975 has size 4, so
pointer at offset 5 is out-of-bounds

specifically for the ptr.write() where ptr is a pointer to ZST.

Somewhat confusing that miri complains about the write, maybe a new
feature.

- Do not allocate when deserializing ArrayString
- Serialize length as u32, not as u64, to be consistent with
  serialization of [T] and str
- Add tests

Is not supported at msrv anyway (msrv applies to defaults)

[ci skip]