fix(rust): Add missing PhantomDatas to BackingStorage

[orlp]

Whenever we have *mut T to actually store an owning reference to T, that struct should also have PhantomData<T>.

https://github.com/rust-lang/rfcs/blob/master/text/0769-sound-generic-drop.md#phantom-data

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.92%. Comparing base (fcf03de) to head (e82247c).
Report is 23 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #18699   +/-   ##
=======================================
  Coverage   79.92%   79.92%           
=======================================
  Files        1513     1513           
  Lines      203329   203340   +11     
  Branches     2892     2892           
=======================================
+ Hits       162501   162520   +19     
+ Misses      40280    40272    -8     
  Partials      548      548           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.