Update kurbo and svg dependencies, fix a clippy warning. #1519

Job	Run time
cargo clippy+test (wasm32) (macOS-11)	2m 31s
cargo clippy+test (macOS-11)	52s
cargo clippy+test (wasm32) (windows-2019)	3m 57s
cargo clippy+test (windows-2019)	2m 46s
cargo clippy+test (wasm32) (ubuntu-latest)	1m 27s
Docs (macOS-11)	2m 56s
cargo clippy+test (ubuntu-latest)	2m 43s
Docs (windows-2019)	2m 47s
Docs (ubuntu-latest)	1m 50s
cargo fmt	16s
cargo test nightly (macOS-11)	1m 31s
cargo test nightly (windows-2019)	1m 43s
cargo test nightly (ubuntu-latest)	2m 0s
	27m 19s

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