refactor: use FutureExt if possible

[Berrysoft]

A small optimization to reduce memory usage.

[Copilot]

Pull request overview

This PR refactors a couple of async call sites to use futures_util::FutureExt combinators (map) instead of creating new async state machines, aligning with the goal of slightly reducing per-future memory overhead in hot paths.

Changes:

• Refactor Runtime::spawn_blocking to map the Submit future output directly rather than wrapping it in an async move block.
• Refactor BufReader::fill_buf’s Buffer::with(...) callback to return a mapped future instead of an async move closure.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
compio-runtime/src/lib.rs	Uses FutureExt::map to avoid an extra async wrapper in spawn_blocking.
compio-io/src/read/buf.rs	Uses FutureExt::map to avoid an extra async closure when refilling the buffer.

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[AsakuraMizu]

Generally LGTM, but just a minor nitpicking: why are the two files importing FutureExt at different locations?

[Berrysoft]

I prefer importing at the top, but there's another FutureExt in compio-runtime. rustc complains about that.