BigInteger type (500 Gold)

[mikera]

Convex should support a native BigInteger numeric type

• Should support key arithmetic operations (+ - * / mod quot abs signum etc.)
• Should be implemented as an ACell for CVM usage
• Should have a canonical encoding that is unique for each BigInteger value
• Should have juice costs that reflect computational complexity of operations
• Should be relatively efficient, e.g. using longs for implementation rather than individual bytes