Similar to properties, subscripts require synthetic construction of
getters and setters. Although subscripts have `get` and `set` handlers,
they can define (multiple) parameters and are parsed as methods, making
it a slight variant on the typical method handling.

```swift
protocol SubscriptedProtocol {
  subscript(row: Int, col: Int) -> String
}
```

The above roughly produces the following mock code:

```swift
class SubscriptedProtocolMock {
  // Mocked implementation
  override subscript(row: Int, col: Int) -> String {
    get {
      // ...
    }
    set {
      // ...
    }
  }

  // Matchable implementation
  func getSubscript(row: Int, col: Int) -> String {
    // ...
  }

  func setSubscript(row: Int, col: Int, newValue: String) {
    // ...
  }
}
```

Which can then be tested with similar syntax to properties. The
parameter `newValue` is named to differentiate between indices and the
value being set.

```swift
// Stubbing
given(myMock.getSubscript(1, 2)) ~> "foobar"

// Verification
verify(myMock.setSubscript(1, 2, newValue: any())).wasCalled()
```