Replace BigInt -> BigUInt conversion code and other occurrences of such code

Improve this conversion process
`BigInt` -> `Float` -> `String` -> `Decimal` -> `String` -> `BigUInt`

TokensCardCoordinator.swift:326
```swift
let ethCostRoundedTo5dp = String(format: "%.5f", Float(string: String(ethCost))!)
let cost = Decimal(string: ethCostRoundedTo5dp)! * Decimal(string: "1000000000000000000")!
let wei = BigUInt(cost.description)!
```