Keep track of residuals in `IterState`, adapted `GaussNewton` accordingly #780

Job	Run time
tests-argmin	2m 15s
clippy	1m 5s
tests-argmin-math	3m 24s
tests-argmin-serde1-feature	34s
cargo-deny (advisories)	22s
docs-build	23s
cargo-deny (bans licenses sources)	29s
rustfmt	18s
wasm-build	26s
	9m 16s

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