perf: eliminate spurious string copies in cpp2rust converter #233

Job	Run time
Run Tests (macos-latest, Release)	43m 25s
Run Tests (ubuntu-latest, Debug)	15m 55s
Run Tests (macos-latest, Debug)	43m 14s
Run Tests (ubuntu-latest, Release)	18m 50s
	2h 1m 24s

Provide feedback