[DRAFT] try to reduce edge cases where BigInts are parsed as BigDecimals first

[pjfanning]

• seems like there are edge cases where BigDecimal conversion to BigInteger is very slow
• this change reduces the chance that a BigInteger is first parsed as a BigDecimal
• doesn't completely disallow this path - just tries to avoid it
• causes Prevent inefficient internal conversion from BigDecimal to BigInteger wrt ultra-large scale #968 failing test to actually fail fast instead of timing out

[cowtowncoder]

@pjfanning Would be happy to merge this (after #969 I think), but is this still WIP?

[pjfanning]

@cowtowncoder this breaks the parsing of 'e' notation BigInts - I think we'll need to address #968 (comment) instead