math/big: add big.Int methods Inc() and Dec()

[lmittmann]

Adding a sample implementation for big.Int methods Inc() and Dec(). #29951

Incrementing and decrementing big.Int's by one can be done with less comparisons and allocations opposed to x.Add(x, one) and x.Sub(x, one).

Benchmark results:

goos: linux
goarch: amd64
pkg: math/big
BenchmarkInc-4         70917309                16.6 ns/op
BenchmarkIncByAdd-4    55380994                18.7 ns/op

goos: linux
goarch: amd64
pkg: math/big
BenchmarkDec-4         63753597                17.3 ns/op
BenchmarkDecBySub-4    57247021                21.3 ns/op

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[mvdan]

The CLA was never signed, and this needs an issue to propose the API changes first. Closing for now.