refactor: break SolInput to its own crate #1678

Job	Run time
miri --no-default-features	10s
miri	13s
test stable --no-default-features	2s
miri --all-features	14s
test stable	3s
feature-checks	6s
test stable --all-features	2s
test beta --no-default-features	9s
wasm	9s
test beta	2s
test beta --all-features	10s
test nightly --no-default-features	11s
clippy	10s
test nightly	2s
docs	10s
test nightly --all-features	10s
fmt	9s
test 1.65 --no-default-features	9s
test 1.65	9s
test 1.65 --features json	6s
	2m 36s

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