Implemented checked multiplication for ValueSums. #560

Job	Run time
Bitrot check	58s
Nightly lint	29s
Lint	45s
Build target wasm32-unknown-unknown	1m 6s
Intra-doc links	1m 20s
Build target wasm32-wasi	41s
Rustfmt	16s
Security - Cargo Audit	3m 15s
Test on ${{ matrix.os }}	0s
	8m 50s

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