Use BigDecimalParser for BigInteger parsing very long numbers

[pjfanning]

Maybe something for jackson 2.15. Relates to #813

String to BigInteger support is currently done using new BigInteger(str).

BigDecimalParser has some performance related code for handling numbers with many digits. #813 suggests that we might be able to get improvements in performance and error handling by using some ideas from jsoniter_scala. BigInteger could probably benefit from these too.

@plokhotnyuk fyi

[pjfanning]

@cowtowncoder could we consider #826 as a late breaking change for Jackson 2.14.0?

[cowtowncoder]

@pjfanning Considering everything else, I think that's safe enough so merged PR to get this improvement in.
I do need to stop merging things of course, but this seemed like non-problematic thing even if last minute.