Introduce Input::builder API

[MichaReiser]

This PR introduces a new Input::builder API.

let input_high = MyInput::builder(&db).durability(Durability::HIGH).new(2200);

It's intentionally limited for now but we could extend it in the future to e.g. allow setting per-field durability.

Design

I'm not super happy with the implementation. Mainly because it requires a private module to hide most implementation details of the Builder.
But even then, we don't get perfect encapsulation because the Builder::create method is still visible in the input's module. But I'm not sure what else we could do.

I had to revert implementing Configuration directly on the $Struct because self-referential types then stopped compiling. Example:

salsa/tests/accumulate-reuse.rs

Lines 16 to 20 in 6975a47

	#[salsa::input]
	struct List {
	value: u32,
	next: Option<List>,
	}

What I dislike about the current solution is that Builder::inner is accessible from the input's module. But not sure what we should do about it. I tried using a trait but that's a bit annoying to use because it must be explicitly imported from the ingredient's scope.

Tests

I did add a test but I haven't figured out a way to automatically verify if a tracked function was validated using deep or shallow comparison.
I did do some manual testing by adding dbg statements that show that Salsa does use the shallow comparison for inputs with Durability::HIGH.

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[MichaReiser]

See #534

[nikomatsakis]

What about a setup like this one?

https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=11175c08bb02381f293d7558dcdae52f

The idea is to have a trait Builder { type Builder; } in Salsa and then to create a module inside the const. You can still name the builder type then by doing <Self as Builder>::Builder. One thing I like about this is that it also means others can name the builder type if they want to.

[MichaReiser]

That sounds interesting. I'll give it a try

[MichaReiser]

I'll re-open this PR from my own fork.