Add option to decode CBOR bignum to interface{} as *big.Int

[fxamacker]

Closes #444

This PR adds decoding option BigIntDecMode which specifies how to decode CBOR bignum to Go interface{}.

• BigIntDecodeValue (default) makes CBOR bignum decode to big.Int.
• BigIntDecodePointer makes CBOR bignum decode to *big.Int.

Default behavior is unchanged to maintain backward compatibility. However, using the new option is recommended as it improves speed, reduces memory use, and is more ergonomic.

Benchmark Comparison

                  │ bench_decode_to_bigint.txt │     bench_decode_to_pbigint.txt      │
                  │           sec/op           │    sec/op     vs base                │
DecodeToBigInt-12                  238.9n ± 8%   196.2n ± 10%  -17.87% (p=0.000 n=20)

                  │ bench_decode_to_bigint.txt │    bench_decode_to_pbigint.txt     │
                  │            B/op            │    B/op     vs base                │
DecodeToBigInt-12                   144.0 ± 0%   112.0 ± 0%  -22.22% (p=0.000 n=20)

                  │ bench_decode_to_bigint.txt │    bench_decode_to_pbigint.txt     │
                  │         allocs/op          │ allocs/op   vs base                │
DecodeToBigInt-12                   5.000 ± 0%   4.000 ± 0%  -20.00% (p=0.000 n=20)

Special Thanks

Many thanks to @extemporalgenome for opening issue #444! 👍

[x448]

LGTM!

Maybe show benchstat comparison for ns/op and alloc/op between these settings.

[fxamacker]

@x448 Thanks! I added benchmark comparisons to the PR text.

[extemporalgenome]

Thank you!