[BigInt tests] ✅🐰 Codable

[LiarPrincess]

Please read the #242 Using tests from “Violet - Python VM written in Swift” before.

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🐰 Discussion

Currently Codable uses description which is a String with radix 10.

Following options may be better:

• String with radix 32
  • ✅ human readable - just like current format; not sure if we care about this property
  • ✅ much shorter in transport - though it may compress a bit worse
  • ✅ faster to encode/decode - though the current implementation is slow
• Binary - just an array of UInt32 with sign
  • ❌ not human readable
  • ✅ ultra compact
  • ✅ ultra fast to encode/decode

We need to remember that BigInt can get really big. In [BigInt tests] 💀 Init from float 754 we operate on integers that have 3000 digits with 32 radix. With radix 10 they would be much longer.