[BigInt tests] 💀 Init from float 754

[LiarPrincess]

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

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💀 Crash

func test_initFromFloat_crash() {
  // 9223372036854775808 = UInt64(1) << Float80.significandBitCount
  let int: UInt64 = 9223372036854775808
  let big = BigInt(int)
  let fromInt = Float80(exactly: int) // works
  let fromBigInt = Float80(exactly: big) // crash
}