Add java/lang/StringBuffer::charAt(I)C #622

Job	Run time
rust_ci (windows-latest, beta)	2m 3s
rust_ci (macos-latest, beta)	1m 17s
rust_ci (windows-latest, stable)	1m 52s
rust_ci (ubuntu-latest, beta)	56s
rust_ci (ubuntu-latest, stable)	1m 29s
rust_ci (macos-latest, stable)	1m 4s
	8m 41s

Provide feedback