Read data map file

[seddonym]

Moves the reading of the cache data file to Rust.

This doesn't make much difference to the performance as it stands, but in a follow up PR we will want to optimize this (probably by moving from JSON to a binary file format) and we might as well do it in Rust.

A large proportion of the time we spend building the graph from a cache is spent reading and deserializing the data cache file - nearly 0.8s of the 1.2s taken to scan this large graph.

The benchmarks indicate a regression, but it doesn't seem to make a significant difference running this locally on a large graph.

Before

After

[codspeed-hq]

CodSpeed Instrumentation Performance Report

Merging #244 will degrade performances by 15.9%

_{Comparing read-data-map-file (b7414a8) with main (e7e1d8d)}

Summary

❌ 3 (👁 3) regressions
✅ 20 untouched benchmarks

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
👁	test_deep_layers_large_graph_kept	17.1 ms	20.4 ms	-15.9%
👁	test_no_chain	1.1 ms	1.2 ms	-11.26%
👁	test_no_chains	1.1 ms	1.2 ms	-11.25%