Move `Batcher::seal` to `Builder` #453

Job	Run time
test mdBook	35s
cargo test on ubuntu, rust stable	42s
cargo test on ubuntu, rust 1.78	54s
cargo test on macos, rust stable	1m 0s
cargo test on macos, rust 1.78	1m 7s
cargo test on windows, rust stable	2m 49s
cargo test on windows, rust 1.78	1m 33s
	8m 40s

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