optimize CodeStream.__iter__

[charles-cooper]

a couple fixes:

• CodeStream.program_counter was a hotspot but not in __slots__
• avoid write-back for local pc variable

What was wrong?

How was it fixed?

Todo:

• [ ] Clean up commit history

• [ ] Add entry to the release notes

Cute Animal Picture

[charles-cooper]

(a bit unclear to me that avoiding the pc writeback is better than having a local variable.)