shred: use RefCell to eliminate mut from random source handling #1422

Job	Run time
Style/Python	5s
Style/spelling (ubuntu-latest, feat_os_unix)	18s
Style/format (ubuntu-latest, feat_os_unix)	16s
Pre-commit hooks	24s
Style/toml	13s
Style/lint (macos-latest, feat_os_macos)	55s
Style/lint (windows-latest, feat_os_windows)	1m 59s
Style/lint (ubuntu-latest, all, true)	1m 26s
	5m 36s

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