move dsl::exists into default feature

[vnghia]

As discussed here: #3936 (reply in thread)

[weiznich]

Thanks for opening this PR 👍

The CI failures are related to rust-lang/rust#120830, which means we just need to wait until a fixed rust nightly version is released.

[vnghia]

I found that our doc is built with i-implement-a-third-party-backend-and-opt-into-breaking-changes enabled so only this exists

diesel/diesel/src/expression/mod.rs

Lines 31 to 32 in dfeec4b

	#[cfg(feature = "i-implement-a-third-party-backend-and-opt-into-breaking-changes")]
	pub mod exists;

is included in the doc and not the other one.

dsl::exists actually exists without i-implement-a-third-party-backend-and-opt-into-breaking-changes and i can use it normally. How should we address this confusion ?

[weiznich]

dsl::exists actually exists without i-implement-a-third-party-backend-and-opt-into-breaking-changes and i can use it normally. How should we address this confusion ?

I would just rename that module to exist_implementation or something like that.

[vnghia]

array_comparison also has this confusion as well. Should I fix in this PR as well ?

Edit: renaming seems to cause a lot of things to fail. I think it's better to wait for rust-lang/rust#1998