Codegen: option to use BTreeMap as map representation #1978

Job	Run time
Check sync-readme	46s
linux beta (default features) (btreemap)	2m 31s
linux nightly (all features) (btreemap)	2m 15s
linux beta (default features) (hashmap)	2m 29s
linux stable (default features) (btreemap)	2m 27s
linux nightly (all features) (hashmap)	2m 14s
linux stable (default features) (hashmap)	2m 23s
linux stable (with-bytes) (btreemap)	1m 11s
linux stable (with-bytes) (hashmap)	1m 8s
mega-linter	2m 20s
rustfmt check	34s
Miri test	42s
CI self-check	17s
windows stable (default features) (btreemap)	2m 26s
windows stable (default features) (hashmap)	3m 8s
	26m 51s

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