chore: Move spark unsafe classes into spark_unsafe

[EmilyMatt]

Which issue does this PR close?

Towards #3554

Rationale for this change

A smaller change, These were just in the main shuffle directory, and are not even specifically tied the shuffle.
Moved them all into a specific directory.

What changes are included in this PR?

Just moved them into a directory, reduced visibility of a couple of functions

How are these changes tested?

By the current tests

[andygrove]

Thanks @EmilyMatt

[andygrove]

@EmilyMatt looks like you need to run cargo fmt on this one

[EmilyMatt]

@andygrove My bad, added the mod statement later

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.99%. Comparing base (f09f8af) to head (dafe646).
⚠️ Report is 919 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #3373      +/-   ##
============================================
+ Coverage     56.12%   59.99%   +3.86%     
- Complexity      976     1464     +488     
============================================
  Files           119      175      +56     
  Lines         11743    16165    +4422     
  Branches       2251     2681     +430     
============================================
+ Hits           6591     9698    +3107     
- Misses         4012     5114    +1102     
- Partials       1140     1353     +213     

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