feat: `FoldAdd` and `FoldMul` (#209) #107

Job	Run time
Test Linux (stable, x86_64)	23s
Lint	23s
Test non-Linux (macos-latest, stable)	27s
Test Linux (stable, i686)	1m 11s
Test Generated	6s
Test non-Linux (macos-latest, stable, --features const-generics)	28s
Test Linux (stable, sparc64)	1m 24s
Test non-Linux (windows-latest, stable)	1m 53s
Test Linux (stable, --features const-generics, x86_64)	23s
Test non-Linux (windows-latest, stable, --features const-generics)	1m 54s
Test Linux (stable, --features const-generics, i686)	1m 12s
Test Linux (stable, --features const-generics, sparc64)	1m 7s
Test Linux (beta, x86_64)	19s
Test Linux (beta, i686)	1m 17s
Test Linux (beta, sparc64)	1m 13s
Test Linux (beta, --features const-generics, x86_64)	23s
Test Linux (beta, --features const-generics, i686)	1m 20s
Test Linux (beta, --features const-generics, sparc64)	1m 14s
Test Linux (nightly, x86_64)	22s
Test Linux (nightly, i686)	1m 12s
Test Linux (nightly, sparc64)	1m 14s
Test Linux (nightly, --features const-generics, x86_64)	23s
Test Linux (nightly, --features const-generics, i686)	1m 12s
Test Linux (nightly, --features const-generics, sparc64)	1m 11s
Test Linux (1.37.0, x86_64)	1m 56s
All Succeeded	2s
	24m 9s

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