feat: add stream::from_fn

[nanoqsh]

This adds an analogue of std::iter::from_fn for streams.

Also creates a new feature async-closure that allows to use newer compiler version 1.85 or later to use async closures. Crates for older versions may use this function without the feature but it'll be more limited.

[taiki-e]

Thanks for the PR. This feature is currently not tested in CI, so could you add cargo test --all-features to the following?

futures-lite/.github/workflows/ci.yml

Lines 56 to 58 in 5c196b9

	- run: cargo test
	- run: cargo test --no-default-features
	- run: cargo test --no-default-features --features alloc

[nanoqsh]

Thanks for the PR. This feature is currently not tested in CI, so could you add cargo test --all-features to the following?

futures-lite/.github/workflows/ci.yml

Lines 56 to 58 in 5c196b9

	- run: cargo test
	- run: cargo test --no-default-features
	- run: cargo test --no-default-features --features alloc

Initially I added ignore to the example with async closure to be able to generate documentation and run tests on Rust earlier than 1.85. The syntax of async closure prevents its use from being hidden behind cfg(feature = "async-closure") attribute, it still won't compile.