Add StringBuffer::setLength #453

Job	Run time
rust_ci (macos-latest, stable)	1m 5s
rust_ci (macos-latest, beta)	1m 22s
rust_ci (macos-latest, nightly)	1m 24s
rust_ci (ubuntu-latest, stable)	59s
rust_ci (ubuntu-latest, beta)	1m 35s
rust_ci (ubuntu-latest, nightly)	1m 33s
rust_ci (windows-latest, stable)	2m 51s
rust_ci (windows-latest, beta)	3m 26s
rust_ci (windows-latest, nightly)	2m 54s
	17m 9s

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