Rollup merge of #75711 - CohenArthur:split-up-astconv, r=oli-obk

Split `astconv.rs` into its own submodule

Fixes #67418

This changed induced a few changes across the Type checker, but only there. Mostly, it was just renaming `Self::` into something else to call specific methods from a subtrait instead of having a 2500+ lines one.

I split up the `astconv.rs` file into its own module. This way, directives such as
```rust
use crate::astconv::AstConv;
```
are still valid, and doing
```rust
use crate::astconv::{AstConv, AstConvGeneric};
```
is possible

(instead of having two modules, one named `astconv_generic.rs` for example and `astconv.rs`)

I'm not entirely sure that the name `AstConvGeneric` is a good one. However, only methods related to lifetimes or generics have been moved over to this module. Sorry about the large diff.

I'd be very happy to make any correction you deem necessary.

r? @oli-obk

0 files changed, 0 insertions, 0 deletions