(!) BigInteger should specify range explicitly

[fulldecent]

BigIntegers are limited in the AVM ABI at:

https://github.com/aionnetwork/AVM/blob/master/org.aion.avm.userlib/src/org/aion/avm/userlib/abi/ABIStreamingEncoder.java#L480

a) This limitation is not specified in documentation. aionnetwork/aion-docs#64
b) The actual limits of the BigIntegers should be mentioned in code for clarity. I think it will be [-2^255, 2^255 - 1]. But the exact number is useful to document.

[fulldecent]

Requesting assistance @jeff-aion

[fulldecent]

Experiments done here fulldecent/aion-aip040@53b77b9

Found that BigInteger can exceed 2^255 but not when encoding to ABI.