Split up `astconv.rs`

[oli-obk]

That file (https://github.com/rust-lang/rust/blob/dab390683196e31116cb61ccf3eec1a4af729def/src/librustc_typeck/astconv.rs) has almost 3k LOC and just 3 lines of documentation on the file (methods and types are documented though). I'm entirely unsure how to process that file. I think splitting it up into submodules (e.g. start by extracting out all functions just handling error reporting into a separate module).

Some of this work can be just mechanical splitting up, but some parts may require you to start understanding at least some of what's going on in the file in order to find a good module structure.

cc @Centril

[TheSamsa]

I'll do it, although I am not familiar with the typeck.
At least the mechanical part is no problem. As to the model structure, I am going to dive into that issue so I can better think of a reasonable structure.

If it is a problem that I am not familiar with the topic, I will back down, no worries.

[oli-obk]

That's awesome. Please wait until #67268 is merged though (you can subscribe to it so you see when it does get merged), because otherwise you'll just get loads of conflicts.

[oli-obk]

If you are unsure about the module structure, you can first post a design suggestion here if you want.

[Centril]

@rustbot assign @TheSamsa

[spadaval]

@TheSamsa You still working on this?

[TheSamsa]

Not quite, my other PR is not yet through and I thought I wait for it.

But if you want to do it, just take it.
If not I'll be on it soon.

[basil-cow]

I would like to claim this, if nobody minds

[oli-obk]

@rustbot assign @Areredify

It's all yours!