Fix panic on encoding map with integer keys

[MarshalX]

No description provided.

[codspeed-hq]

CodSpeed Performance Report

Merging #60 will degrade performances by 26.57%

_{Comparing fix-panic-int-key (55d1c95) with main (a3b2a64)}

Summary

⚡ 5 improvements
❌ 4 regressions
✅ 183 untouched benchmarks

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

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
⚡	test_dag_cbor_decode[roundtrip07.json]	17 µs	15.3 µs	+11.11%
⚡	test_dag_cbor_decode[roundtrip22.json]	22.8 µs	17.1 µs	+33.39%
❌	test_dag_cbor_decode[roundtrip23.json]	17 µs	23.1 µs	-26.57%
❌	test_dag_cbor_encode[roundtrip04.json]	17 µs	22.7 µs	-25.12%
⚡	test_dag_cbor_encode[roundtrip06.json]	21.6 µs	16 µs	+34.69%
❌	test_dag_cbor_encode[roundtrip08.json]	17 µs	22.6 µs	-24.88%
❌	test_dag_cbor_encode[roundtrip12.json]	18.7 µs	22.8 µs	-17.79%
⚡	test_dag_cbor_encode[roundtrip16.json]	22.4 µs	17.2 µs	+30.47%
⚡	test_dag_cbor_encode[roundtrip25.json]	22.3 µs	17.6 µs	+26.46%