Count computed property

[laclouis5]

Currently the count property is derived from the Sequence default implementation which is O(n). I suggest implementing the count of a PythonObject as the value returned by Python.len() :

• It will be faster to access the count of an object
• It will fail is the object has no length

This change could speed-up code that relies on indices and use count :

import PythonKit

let array = [Int](repeating: 0, count: 1_000_000_000).pythonObject
let half = array.count / 2  // Currently a O(n) operation
let elements = array[..<half]

Maybe other performance critical Sequence conformances could be supplied as well, such as underestimatedCount, or Collection's distance().