feat: well founded stream class

[digama0]

Based on discussions on Zulip. Still needs more love to make it polished.

[fgdorais]

Just wondering what the status is here: is it deadlocked? on the backburner? abandoned?

[digama0]

It's please-adopt. I'm not sufficiently invested to fix the remaining issues (or really recall what they are). There is some concern that this adds yet another way to write a for loop, which means we need more lemmas to relate the different options. Also the StreamAccType stuff is nasty, it needs core changes so that forIn can depend on the value and not just the type.