feat: split Chain into a struct and ChainKind enum #16

Job	Run time
feature-checks	23s
fmt	16s
clippy	12s
docs	13s
miri --no-default-features	36s
miri	37s
test stable --no-default-features	10s
wasm	16s
test stable	10s
test stable --all-features	16s
miri --all-features	43s
test beta --no-default-features	12s
test beta	19s
test beta --all-features	25s
test nightly --no-default-features	12s
test nightly	13s
test nightly --all-features	17s
test 1.65 --no-default-features	21s
test 1.65	16s
	6m 7s

Provide feedback