feat: add `mailmap::Snapshot::iter()` #4385

Job	Run time
test	22m 29s
test-32bit (armv7-linux-androideabi)	1m 8s
test-fast (windows-latest)	13m 55s
test-fast (macos-latest)	2m 48s
test-fast (ubuntu-latest)	3m 4s
check-packetline (ubuntu-latest)	8s
cargo-deny (advisories)	32s
cargo-deny (bans licenses sources)	35s
installation (win-msvc)	6m 31s
installation (win-gnu)	7m 20s
lint	4m 5s
installation (win32-msvc)	7m 21s
installation (win32-gnu)	6m 18s
pure-rust-build	1m 56s
WebAssembly (wasm32-unknown-unknown)	1m 0s
WebAssembly (wasm32-wasi)	30s
	1h 19m 40s

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