Avoid type size explosion when expanding types

[JukkaL]

If TypedDict A has multiple items that refer to TypedDict B, don't duplicate the types representing B during type expansion (or generally when translating types). If TypedDicts are deeply nested, this could result in lot of redundant type objects.

Example where this could matter (assume B is a big TypedDict):

class B(TypedDict):
    ...

class A(TypedDict):
    a: B
    b: B
    c: B
    ...
    z: B

Also deduplicate large unions. It's common to have aliases that are defined as large unions, and again we want to avoid duplicating these unions.

This may help with #17231, but this fix may not be sufficient.

[github-actions]

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

[hauntsaninja]

I ran this commit against mypy_primer with at compile level 3 using an average of 5 runs

This made https://github.com/vega/altair 6.4x faster, going from 110s to 17s!

(all other meaningful projects are in the 0.95x to 1.05x range)

[JukkaL]

Nice, thanks for looking into the perf impact! And I think we still have a bunch of improvements we can make to very large types.