Add java/lang/StringBuffer::length()I #621

Job	Run time
rust_ci (macos-latest, beta)	41s
rust_ci (ubuntu-latest, stable)	33s
rust_ci (windows-latest, stable)	31s
rust_ci (ubuntu-latest, beta)	28s
rust_ci (windows-latest, beta)	32s
rust_ci (macos-latest, stable)	33s
	3m 18s

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