Add support for `reserved` keyword in enums #1993

Job	Run time
linux stable (default features)	2m 22s
linux stable (with-bytes)	1m 19s
Check sync-readme	42s
mega-linter	2m 21s
linux beta (default features)	2m 34s
Miri test	38s
rustfmt check	38s
CI self-check	21s
windows stable (default features)	4m 3s
linux nightly (all features)	2m 24s
	17m 22s

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