Add compactMapValues to Dictionary

Evolution Pitches
15

Thank you lorentey!

I test performance this with my implementation as below

extension Dictionary {
    public func compactMapValues<T>(_ transform: (Value) throws -> T?) rethrows -> [Key: T] {
        return try self.reduce(into: [Key: T](), { (result, x) in
            if let value = try transform(x.value) {
                result[x.key] = value
            }
        })
    }
}

Then I found latter is more faster so that I'll implement by mine and try to fix SPM too.