Change return type of root_instrument to Instrument

[jenshnielsen]

The point of root_instrument is to get to the instrument.

• Breaking change notice

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 69.32%. Comparing base (86273d9) to head (269ad6e).
⚠️ Report is 10 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7924      +/-   ##
==========================================
- Coverage   70.30%   69.32%   -0.98%     
==========================================
  Files         333      333              
  Lines       32401    32401              
==========================================
- Hits        22778    22461     -317     
- Misses       9623     9940     +317     

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