Added functions from node/vms/dexvm/math:
- Div64: safe division with error on zero
- MulBig: multiply uint64s returning big.Int
- MulDiv64: (a * b) / c without overflow
- MulDivRoundUp64: ceiling division variant
- BigMulDiv: big.Int multiplication+division
- BigMulDivRoundUp: big.Int ceiling variant
- Clamp: generic ordered type clamping
Also added common big.Int values (bigZero, bigOne, maxUint64Big)
for performance in division operations.
zeekay