feat: `FoldAdd` and `FoldMul` #106

Job	Run time
Lint	20s
Test Generated	5s
Test non-Linux (macos-latest, stable)	27s
Test non-Linux (macos-latest, stable, --features const-generics)	28s
Test Linux (stable, x86_64)	16s
Test non-Linux (windows-latest, stable)	48s
Test Linux (stable, i686)	1m 10s
Test non-Linux (windows-latest, stable, --features const-generics)	1m 26s
Test Linux (stable, sparc64)	1m 12s
Test Linux (stable, --features const-generics, x86_64)	16s
Test Linux (stable, --features const-generics, i686)	1m 20s
Test Linux (stable, --features const-generics, sparc64)	1m 9s
Test Linux (beta, x86_64)	23s
Test Linux (beta, i686)	1m 18s
Test Linux (beta, sparc64)	1m 12s
Test Linux (beta, --features const-generics, x86_64)	23s
Test Linux (beta, --features const-generics, i686)	1m 14s
Test Linux (beta, --features const-generics, sparc64)	1m 14s
Test Linux (nightly, x86_64)	21s
Test Linux (nightly, i686)	1m 13s
Test Linux (nightly, sparc64)	1m 16s
Test Linux (nightly, --features const-generics, x86_64)	23s
Test Linux (nightly, --features const-generics, i686)	1m 15s
Test Linux (nightly, --features const-generics, sparc64)	1m 17s
Test Linux (1.37.0, x86_64)	1m 53s
All Succeeded	2s
	22m 21s

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