Checked add and sub for `ValueSum` #480

Job	Run time
Bitrot check	1m 8s
Bitrot check	1m 8s
Nightly lint	45s
Nightly lint	45s
Lint	51s
Lint	51s
Build target wasm32-unknown-unknown	47s
Build target wasm32-unknown-unknown	47s
Intra-doc links	55s
Intra-doc links	55s
Build target wasm32-wasi	44s
Build target wasm32-wasi	44s
Rustfmt	13s
Rustfmt	13s
Security - Cargo Audit	4m 1s
Security - Cargo Audit	4m 1s
Test on ubuntu-latest	3m 26s
Test on ubuntu-latest	3m 26s
Test on windows-latest	4m 59s
Test on windows-latest	4m 59s
Test on macOS-latest	4m 24s
Test on macOS-latest	7m 32s
	47m 34s

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