move heap traits to separate file

[davidhewitt]

In preparation for #207, this moves traits which aren't necessary for heap soundness into a separate file.

[codspeed-hq]

Merging this PR will not alter performance

✅ 13 untouched benchmarks

---

_{Comparing dh/guard-macro (57b1796) with main (dc28860)}

[codecov]

Codecov Report

❌ Patch coverage is 93.33333% with 5 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
crates/monty/src/heap_traits.rs	93.33%	5 Missing ⚠️

📢 Thoughts on this report? Let us know!