Merge pull request #704 from frankmcsherry/flat_map_builder #1

Job	Run time
Cargo clippy	28s
cargo test on macos, rust 1.79	3m 12s
cargo test on ubuntu, rust 1.79	2m 5s
cargo test on macos, rust stable	2m 38s
cargo test on windows, rust 1.79	3m 38s
cargo test on windows, rust stable	3m 24s
cargo test on ubuntu, rust stable	1m 44s
	17m 9s

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