`<generator>`: Cleanup `P2502R2_generator/test.cpp`

[JMazurkiewicz]

• Add using namespace std
• Rename some functions (e.g. zip -> co_zip, meow -> co_upto)
• Add missing includes
• Other minor changes (see commit messages)

[StephanTLavavej]

Thanks, merged! :heart_eyes_cat: :rocket: Noticed a tiny pre-existing issue, can easily be cleaned up later.