Remove Allocations from Panic Handler

[adamperlin]

This PR fixes #735. The approach used here is to create a FixedStringBuf type which wraps a byte slice and which implements fmt::Write by writing into this underlying slice. This static buffer can then be used to format the actual panic message.

[Copilot]

Pull Request Overview

This PR addresses issue #735 by removing allocations from the panic handler in the hyperlight guest environment. The implementation introduces a FixedStringBuf type that wraps a pre-allocated byte slice and implements fmt::Write to enable string formatting without dynamic allocation.

Key changes:

• Implements a new FixedStringBuf type for allocation-free string formatting
• Replaces allocation-based panic message handling with a static buffer approach
• Adds test coverage to verify panic handling works correctly under memory exhaustion scenarios

Reviewed Changes

Copilot reviewed 6 out of 10 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
src/hyperlight_common/src/fixed_buf.rs	New module implementing FixedStringBuf for allocation-free string formatting
src/hyperlight_common/src/lib.rs	Adds the new fixed_buf module to the crate
src/hyperlight_guest_bin/src/lib.rs	Replaces allocation-based panic handler with static buffer implementation
src/tests/rust_guests/simpleguest/src/main.rs	Adds heap exhaustion test function to verify panic behavior under OOM conditions
src/hyperlight_host/tests/integration_test.rs	Adds integration test to verify panic handler doesn't cause stack overflow
flake.nix	Updates cargo hash for build system