Possible naming suggestion: lastIndex(of: )

Possible implementation:

extension BidirectionalCollection where Iterator.Element : Equatable {
    // RFE: https://bugs.swift.org/browse/SR-1504
    public func lastIndex(of element: Iterator.Element) -> Index? {
        var i = self.endIndex
        while i != self.startIndex {
            self.formIndex(before: &i)
            if self[i] == element {
                return i
            }
        }
        return nil
    }
}

This would need to go through the Swift Evolution Process.

This is implemented as of Swift 4.2.

@natecook1000 The docs now say there is a `firstIndex` and a `lastIndex` but no `index`. Yet there is no compile error when using `index`. Shouldn't this be a warning at least? We should all be migrating to `firstIndex` but nothing calls out the change. I've been using Swift 4.2 since WWDC and I only just noticed this by accident.

Proposal: SE-0204