Don't reference std in generated code from #[instrument]

[bitdriftr]

Motivation

This change allows using the #[instrument] macro in #![no_std] crates.

Solution

The macro was generating code referencing ::std::boxed::Box, but Box can be imported from ::alloc::boxed::Box instead which does not require std.

[Copilot]

Pull Request Overview

Updates the #[instrument] macro to use ::alloc::boxed::Box instead of ::std::boxed::Box to enable usage in #![no_std] crates.

• Replaces std reference with alloc reference in generated code
• Enables #[instrument] macro compatibility with no_std environments

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