There is unsound unsafe code in arena-rs

[A1-Triard]

1. The following code in arena-rs is unsound, because there is no guarantee, that zero-initialized data is a properly initialized T and dereferencing non-properly initialized data is an undefined behavior:

   unsafe {
       *ptr = value;
       // ...
   }

   The *ptr = value line should be replaced with std::ptr::write(ptr, value).

2. There is no guarantee that T object is properly aligned and thus there is an another undefined behavior because only a pointer to a properly aligned object can be dereferenced without undefined behavior.

   Arena should use std::mem::align_of to obtain information about T alignment, and insert appropriate padding before T.