Store InputStreamReader charset name and recreate Decoder per call #677

Job	Run time
rust_ci (windows-latest, beta)	2m 53s
rust_ci (macos-latest, beta)	1m 28s
rust_ci (ubuntu-latest, stable)	1m 17s
rust_ci (windows-latest, stable)	2m 48s
rust_ci (ubuntu-latest, beta)	1m 24s
rust_ci (macos-latest, stable)	1m 4s
	10m 54s

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