Implement cookie prefixes via 'PrefixedJar'. #91

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Job	Run time
Check MSRV	6m 57s
Linux (stable)	3m 21s
Linux (beta)	3m 15s
Linux (nightly)	4m 23s
Windows (stable)	4m 50s
Windows (beta)	4m 57s
Windows (nightly)	4m 31s
	32m 14s