Implement Neo4j vs Doublets benchmark in Rust

[konard]

Summary

This PR implements a Rust benchmark comparing Neo4j database performance against LinksPlatform's Doublets on basic database operations.

Features

• Neo4j client using HTTP Transaction API with raw TCP sockets (compatible with nightly-2022-08-22)
• Benchmark operations: create, update, delete, each_all, each_incoming, each_outgoing, each_concrete, each_identity
• GitHub Actions workflow with Neo4j service container
• Visualization script (out.py) for generating benchmark graphs
• Auto-commit of benchmark results to repository

Structure

Based on the PostgreSQL comparison benchmark template:

• rust/ - Main Rust project with benchmarks
• rust/src/client.rs - Neo4j HTTP client implementation
• rust/benches/ - Criterion benchmark files
• rust/out.py - Python visualization script
• .github/workflows/rust.yml - CI workflow

Technical Notes

• Uses nightly-2022-08-22 Rust toolchain (required by doublets-rs)
• Implements Links and Doublets traits for Neo4j client
• All dependencies pinned to versions compatible with the old nightly toolchain
• Uses raw TCP/HTTP instead of crate dependencies to avoid version conflicts

Test Plan

• Local build compiles successfully with cargo check --benches
• GitHub Actions workflow runs and completes
• Benchmarks execute against Neo4j service container
• Visualization graphs are generated
• Results are auto-committed to repository

Fixes #1

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[konard]

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