Signed integers: Int64, Int128, Int256, Int512

[webmaster128]

Those types should

• Use string serialization in JSON
• Have the common signed integer range −2⁽ⁿ⁻¹⁾ to 2⁽ⁿ⁻¹⁾ − 1
• Only have one zero

Implementation options are more or less those two paths:

1. Use [u64; 8] or [i128; 4] and implement all the math ourselves
2. Rely on i128, U256 and U512 (from uint) to implement math