Find first index of occurrence of a collection in a collection.

BidirectionalCollection+firstIndex.swift

/// Usage: myString.firstIndex(of: otherString)
extension BidirectionalCollection where Element: Equatable {
    func firstIndex(of other: Self) -> Index? {
        guard
            let start = other.first.flatMap(self.firstIndex(of:)),
            self[start...].count >= other.count,
            case let end = self.index(start, offsetBy: other.count),
            zip(self[start ..< end], other).allSatisfy(==)
        else
        {
            return nil
        }

        return start
    }
}