Add support for `reserved` keyword in enums. #1984

Job	Run time
linux stable (default features)	2m 24s
Check sync-readme	44s
linux beta (default features)	2m 32s
linux stable (with-bytes)	1m 16s
linux nightly (all features)	2m 20s
mega-linter	2m 24s
Miri test	36s
rustfmt check	1m 1s
windows stable (default features)	2m 31s
CI self-check	4s
	15m 52s

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