merge: micro-optimize `Merge`, clean up constructors #7739

Job	Run time
cargo-deny (advisories)	23s
build (ubuntu-latest)	4m 38s
cargo-deny (bans licenses sources)	38s
build (macos-13)	6m 57s
build (windows-latest)	7m 52s
Check protos	1m 9s
build (ubuntu-latest, --all-features)	10m 23s
Clippy check	2m 17s
Check that MkDocs can build the docs	22s
Check that MkDocs can build the docs with Poetry 1.3.2	46s
Check formatting	23s
	35m 48s

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