fix: use autocfg to detect rustc version

[AsakuraMizu]

Close #6

[Copilot]

Pull request overview

This PR implements automatic detection of Rust compiler version capabilities using the autocfg crate to conditionally use newer APIs in BorrowedBuf. The changes enable compatibility across different Rust nightly versions and stable by detecting whether BorrowedBuf::init_len() and related methods are available.

Key changes:

• Adds build-time feature detection for borrowedbuf_init capability using autocfg
• Implements fallback code paths for older Rust versions that lack the newer BorrowedBuf API methods
• Extends CI matrix to test against multiple Rust toolchain versions (nightly, specific nightly dates, and stable)

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
build.rs	Adds autocfg-based feature detection to probe for BorrowedBuf::init_len availability
Cargo.toml	Adds autocfg as a build dependency
src/lib.rs	Implements conditional compilation with separate code paths for new and old BorrowedBuf APIs, adding support for the maybe_uninit_fill feature when newer APIs aren't available
.github/workflows/ci.yml	Expands CI matrix to test multiple Rust versions and removes target-specific testing in favor of feature-powerset testing

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