no in-place mutation for str and bytes

[davidhewitt]

It doesn't make sense to mutate these types, they are immutable.

[codspeed-hq]

Merging this PR will degrade performance by 16.12%

❌ 1 regressed benchmark
✅ 12 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
❌	loop_mod_13__monty	270.8 µs	322.8 µs	-16.12%

---

_{Comparing dh/immutable-str (71bc46f) with main (fe9f22a)}

[codecov]

Codecov Report

❌ Patch coverage is 53.33333% with 7 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
crates/monty/src/heap.rs	0.00%	2 Missing ⚠️
crates/monty/src/types/list.rs	0.00%	2 Missing ⚠️
crates/monty/src/types/str.rs	60.00%	2 Missing ⚠️
crates/monty/src/bytecode/vm/binary.rs	0.00%	0 Missing and 1 partial ⚠️

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