refactor and update toolchain to nightly-2025-12-12

[AsakuraMizu]

See the overview generated by Copilot.

TODO

• We use a lot of code from https://github.com/rust-lang/rust, and it's primarily distributed under the terms of both the MIT license and the Apache License (Version 2.0). We need to check if it's compatible with our current license.

[Copilot]

Pull request overview

This PR performs a major refactor of the axio library, transforming it from a minimal std::io-like trait library into a more comprehensive I/O framework for no_std environments. The refactor significantly expands functionality by adding utility types (Empty, Sink, Repeat, Take, Chain, Cursor), buffered writers (BufWriter, LineWriter), enhanced BufReader, specialized copy implementations, and comprehensive test coverage.

Key Changes:

• Complete restructuring of the codebase with modular organization (read/, write/, seek/, utils/, buffered/)
• Addition of utility I/O types and adapters comparable to std::io
• Implementation of BufWriter and LineWriter for buffered writing
• Enhanced BufReader with optimized read operations and seeking support
• Comprehensive test suite with ~2800 lines of new tests
• CI improvements with multi-toolchain testing

Reviewed changes

Copilot reviewed 38 out of 38 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tests/utils.rs	Tests for utility types (empty, sink, repeat)
tests/io.rs	Comprehensive I/O trait tests with 696 lines
tests/impls.rs	Benchmark tests for slice and vector I/O operations
tests/cursor.rs	Tests for Cursor type with 425 lines
tests/copy.rs	Tests for copy specializations
tests/buffered.rs	Extensive buffered I/O tests with 978 lines
src/lib.rs	Refactored entry point with modular exports
src/read/*	Complete Read trait implementation with impls for common types
src/write/*	Write trait with implementations
src/seek/*	Seek trait and implementations
src/utils/*	Utility types (Chain, Cursor, Empty, Repeat, Sink, Take, copy)
src/buffered/*	BufReader, BufWriter, LineWriter implementations
src/prelude.rs	Updated module path references
Cargo.toml	Version bump to 0.2.0, new dependencies
.github/workflows/ci.yml	Multi-toolchain CI testing
build.rs	Nightly version detection for conditional compilation
README.md	Updated description

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[Copilot]

Pull request overview

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

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[AsakuraMizu]

This PR is too big, I will break it down into several smaller ones.