Deduplicate global implementation

[t4c1]

Deduplicate buffer and USMcodepaths in global implementation.

Checklist

Tick if relevant:

• [N/A] New files have a copyright
• [N/A] New headers have an include guards
• API is documented with Doxygen
• [N/A] New functionalities are tested
• Tests pass locally
• Files are clang-formatted

[hjabird]

The inlining of the code makes this a challenging game of spot-the-difference. Can you draw our attention to any important changes? Thanks!

[t4c1]

Inlining was done in a separate commit, so you can look just at deduplication if you want.

[t4c1]

Also, I would say there are no really important changes. Maybe you can give the new variables I created from function parameters a look and check if their names look good.

[hjabird]

LGTM.