Move `Batcher::seal` to `Builder` (#546) #454

Job	Run time
test mdBook	42s
cargo test on ubuntu, rust stable	44s
cargo test on ubuntu, rust 1.78	49s
cargo test on macos, rust stable	1m 13s
cargo test on macos, rust 1.78	50s
cargo test on windows, rust stable	2m 25s
cargo test on windows, rust 1.78	1m 43s
	8m 26s

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