Crash. BInt can't create really big ints

[maximuskravchenko]

STR: Wei(hex: "56d49825b27744c60") (equivalent to 100.10868911999734896 Ether)

Result: Fatal error: Not enough bits to represent a signed value.

Same result with all values bigger then hex 0x56d49825b27744c60.

[DmitryBespalov]

iOS 10.0 also crashing, for example:

BInt("15823297914388465068645274956031579191506355248080856511104898257696315269079", radix: 10)

on iOS 11.0 it works fine.