Post demystifying `IndexByType`

[Andarist]

This is a strong draft of the post - it needs to be polished before we can publish it.

I think it has the potential of splitting into two parts:

1. typegen intro: outline the problem, describe the goals (stuff up to the "indexing" header)
2. IndexByType demystified (the big part from the "indexing" header)

We could make this whole thing a series of blog posts about the typegen's internal machinery. Not every blog post has to be as long as this one - some tricks used in there are much simpler, or at least they can be described much quicker. And I think that each blog post should be its own thing, it's not worth squeezing two different tricks into a single post or something like that.

I would be very grateful for some initial feedback and help with the structure, language, etc

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