Add support in `TokenBuffer` for lazily decoded (big) numbers

[cowtowncoder]

(note: for background see #3721)

There are a few issues with TokenBuffer regarding number coercions, mostly due to eager buffering sometimes needed, which forces decision on use of type (double vs BigDecimal in particular) before knowing target type.
There are also concerns about overhead in some cases, preventing efficient skipping of big number values in cases where buffering is needed: for example, when @JsonCreator use requires preservation of content that would ultimately be skipped.

An idea that came to mind is that since TokenBuffer stores all buffered content using 2 elements -- JsonToken for type, and opaque Object (depending on type) -- it would be possible to augment handling to use hypothetical BufferedNumber (or LazyNumber) type, which could:

1. Store more information on actual NumberType (to preserve distinction between finer grain JsonParser exposes)
2. Allow deferred decoding esp. for "big" numbers (BigInteger, all floating-point types)

This would require support from jackson-core for:

• New type itself (BufferedNumber) as well as implementation(s) if not concrete
• New accessor(s) from JsonParser -- either specific "always get buffered" or hybrid (buffered if not decoded; Number if decoded)

There are probably other subtleties too but this is the general idea. If anyone plans to work on this, another issue needs to be created on jackson-core side.

[pjfanning]

@cowtowncoder I added lazy nums in #3727

In the test case that shows the unnecessary parseBigInteger call now doesn't end up parsing the big int - the LazyBigInteger is stored but the number is never parsed.

Unfortunately, not many existing tests lead to lazy nums being actually parsed - so I can't yet guarantee that the lazy code actually works all the way through.

[cowtowncoder]

Another potential mechanism to use: FasterXML/jackson-core#902 (assuming I get to implement it).

EDIT: Ok it was easier than I thought; getNumberValueDeferred() PR is here:

https://github.com/FasterXML/jackson-core/pull/903

ready for review.

[cowtowncoder]

Ok, instead I went with:

FasterXML/jackson-core#902

with adds JsonParser.getNumberValueDeferred(). I hope to work on this soon, but @pjfanning if you have time and interest don't let me block your progress.

One thing I need to figure out has to do with testing, how to verify expected handling improvements both wrt skipping of unused contents (need mocking, maybe?) and wrt retaining of exact representation through buffering.

[pjfanning]

@cowtowncoder #3761 is open

[cowtowncoder]

Merged #3761 so this is now implemented.