Add support for `reserved` keyword in enums. #1983

Job	Run time
linux nightly (all features)	7m 1s
Check sync-readme	45s
linux stable (default features)	7m 22s
linux beta (default features)	7m 12s
linux stable (with-bytes)	6m 20s
mega-linter	2m 25s
Miri test	38s
rustfmt check	5m 40s
CI self-check	6s
windows stable (default features)	2m 50s
	40m 19s

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