feat: str.join constant folding support (and bytes.join for mypyc) [1/3]

[BobTheBuidler]

Intent:

Mypyc can currently handle constant folding of f-strings, but mypy itself cannot.

Changes:

• implements partial support for constant folding str.join expressions. It is a critical first step for implementing f-string support, which will come in a follow-up PR.
• I've also implemented this for mypyc once I noticed mypyc didn't benefit from this new constant fold ability.
• Lastly, I implemented mypyc constant folding for bytes.join, since mypy constant folding does not support bytes, it feels too similar to str.join to not do ever, and does not feel important enough for its own PR after this one.
  I can break those changes up if you'd prefer.

[BobTheBuidler]

I don't see any reason for this test to not fail based on the code changes. I think I might be using TupleType incorrectly here, or accessing the type in a less-than-optimal way. Any advice here would be appreciated.

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅