Optimize string encoding

[MarshalX]

explanation: read_str performs from_utf8 in rust side which is not necessary because performed from CPython side too

todo: investigate possibilities of invalid utf8 strings here and how it be handled

[codspeed-hq]

CodSpeed Performance Report

Merging #41 will improve performances by 12.68%

_{Comparing optimize-string-encoding (1dbfbbc) with main (1c0df5e)}

Summary

⚡ 3 improvements
✅ 189 untouched benchmarks

Benchmarks breakdown

	Benchmark	main	optimize-string-encoding	Change
⚡	test_dag_cbor_decode_fixtures[garbage-06]	91 µs	82.2 µs	+10.61%
⚡	test_dag_cbor_decode_fixtures[garbage-09]	117.6 µs	104.7 µs	+12.32%
⚡	test_dag_cbor_decode_fixtures[garbage-23]	75.5 µs	67 µs	+12.68%

[MarshalX]

now it's even better!