Add string lastindexof #575

Job	Run time
rust_ci (ubuntu-latest, stable)	59s
rust_ci (macos-latest, stable)	1m 5s
rust_ci (windows-latest, beta)	3m 48s
rust_ci (windows-latest, stable)	4m 44s
rust_ci (macos-latest, beta)	1m 25s
rust_ci (ubuntu-latest, beta)	1m 34s
	13m 35s

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