refactor: add I/O utils & unit tests

[AsakuraMizu]

Depends: #8 #9

This is the third stage of refactoring, extracted from #4.

Changes

• Add common I/O utils, including Chain/copy/Cursor/empty/repeat/sink/Take
• Add unit tests

[Copilot]

Pull request overview

This PR implements the second stage of refactoring for the axio library, adding common I/O utilities including Chain, copy, Cursor, empty, repeat, sink, and Take. The changes enhance basic I/O traits and improve the project structure by organizing code into modules and upgrading to Rust 2024 edition.

Key Changes:

• Added comprehensive I/O utilities (Chain, Copy, Cursor, Empty, Repeat, Sink, Take)
• Implemented BufWriter and LineWriter for buffered writing
• Enhanced BufReader with improved buffer management using Box<[MaybeUninit<u8>]> or heapless::Vec
• Added extensive unit tests and benchmarks
• Upgraded to Rust 2024 edition and updated dependencies

Reviewed changes

Copilot reviewed 35 out of 35 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tests/utils.rs	Unit tests for Empty, Repeat, and Sink utilities
tests/io.rs	Comprehensive tests for core I/O operations including read/write/seek
tests/impls.rs	Benchmarks for slice and Vec read/write implementations
tests/cursor.rs	Tests for Cursor functionality with various backing types
tests/copy.rs	Tests for copy function and its specializations
tests/buffered.rs	Extensive tests for BufReader, BufWriter, and LineWriter
src/lib.rs	Refactored library entry point with modular organization
src/write/mod.rs	Write trait definition and default implementations
src/write/impls.rs	Write trait implementations for various types
src/read/mod.rs	Read and BufRead trait definitions with utilities
src/read/impls.rs	Read trait implementations for slices, VecDeque, etc.
src/seek/mod.rs	Seek trait definition
src/utils/*.rs	Utility types (Chain, Copy, Cursor, Empty, Repeat, Sink, Take)
src/buffered/*.rs	Buffered I/O implementations
Cargo.toml	Updated edition to 2024, dependencies to axerrno 0.2
build.rs	Added maybe_uninit_slice feature detection
README.md	Enhanced documentation with features and limitations

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