NSArray: add [safe:] subscripts

[orchetect]

NSArray: add [safe:] subscripts

[orchetect]

Currently, these are implemented in the library as:

NSArray:

let array = NSArray()

let getVal = nsArray[safe: 1]

NSMutableArray:

let array = NSMutableArray()

// both getters work ([safe:] is inherited from NSArray):
let getVal = array[safe: 1]
let getVal = array[safeMutable: 1]

// setter:
array[safeMutable: 1] = newValue

But the goal is to unify them into a single [safe:] subscript for both NSArray (get) and NSMutableArray (get and set).