Change behavior for ReaderFlags.BIGNUM_AS_RAW and ReaderFlags.NUMBERS_AS_RAW

[MichaelSquires]

This PR makes the following changes:

• Change behavior of ReaderFlags.BIGNUM_AS_RAW:
  • Old behavior: Return big numbers (>= 2**64, < -2**63) as Decimals.
  • New behavior: Return big numbers (>= 2**64, < -2**63) as python long integers.
• Change behavior of ReaderFlags.NUMBERS_AS_RAW:
  • Old behavior: Return all numbers as Decimals.
  • New behavior: Return all numbers as python long integers.
• Added tests to validate behavior of serializing/deserializing numbers with various flags.

Note: These changes are not backward compatible and will probably require a major version bump.

[MichaelSquires]

Hi, I realize this is a potentially controversial change and I'm glad to discuss it. Basically, we needed to be able to deserialize large integers without the Decimal() wrapper. This seemed like a good place/way to split it. Hopefully you agree :)