Speedup new backend

[saulshanabrook]

Speedup new backend by using only one thread and a custom memory allocator Copied from egraphs-good/egglog#637

[codspeed-hq]

CodSpeed Instrumentation Performance Report

Merging #311 will improve performances by 15.92%

_{Comparing speedup-new-backend (4a43cd8) with main (528b3e9)}

Summary

⚡ 1 improvements
✅ 6 untouched benchmarks

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
⚡	test_jit[lda]	7.8 s	6.7 s	+15.92%