Add support for `reserved` keyword in enums #1994

Job	Run time
linux stable (default features)	2m 15s
linux nightly (all features)	2m 20s
linux beta (default features)	2m 26s
linux stable (with-bytes)	1m 19s
windows stable (default features)	3m 15s
Check sync-readme	51s
mega-linter	2m 21s
Miri test	40s
rustfmt check	42s
CI self-check	8s
	16m 17s

Provide feedback