Add Error::new_with_backtrace

[sb123sb123]

Closes #462

Summary

Error::new captures or discovers a backtrace while constructing the error, but stable Rust does not provide a way to attach a backtrace that was captured earlier. This is needed by callers that capture a backtrace before constructing their anyhow error.

This adds Error::new_with_backtrace(error, backtrace) under the existing std feature. It stores the supplied backtrace through the existing error construction path, preserving the existing error chain and default Error::new behavior. A regression test verifies that the supplied backtrace is retained.

Tests

• cargo fmt --all -- --check
• cargo test (including 26 doctests)
• cargo clippy --all-targets -- -D warnings
• cargo doc --no-deps
• cargo check --all-features
• cargo check --no-default-features
• cargo +nightly test --test test_backtrace -- --nocapture
• cargo +nightly check --no-default-features

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