Refactor BufferInfo::Operation as enum class.

[1uc]

Since it's internal we should be free to change this.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (1299c55) 85.94% compared to head (0915008) 85.94%. Report is 3 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #922 +/- ## ========================================== - Coverage 85.94% 85.94% -0.01% ========================================== Files 89 89 Lines 5864 5862 -2 ========================================== - Hits 5040 5038 -2 Misses 824 824 ```

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