Fix compiler failure in Xcode 11 (#22)

* Fix compiler failure in Xcode 11

For some unknown reason (at least to me) the `subscript(position:)`
extension on `MutableCollection` where `Index == Int` was causing the
compilation to fail with a weird `DESERIALIZATION FAILURE` error,
as mentioned on #15.

Updating the `subscript` to use `C.Index` instead of `Int` appears to
make the compiler happy and successfully compile the project. My
suspicion is that it somehow enables it to disambiguate between the
multiple `subscript` implementations, but it's pure speculation 🔮.

On our tests this seems to have fixed the issue, and since it's an
equivalent definition of the subscript (even though more "precise"
from a generics POV), it should cause no side effects.

Fixes #15. 🎉

* Make `NonEmpty` conform to `Collection` in type declaration

Apparently by defining `NonEmpty` as conforming to `Collection` right
away in its declaration (instead of in an extension), the compilation
failure seems to be fixed.

Even when performing a clean build, the target now successfully
compiles. 🎉

Not sure about the actual cause for this, but this definitely seems
like a compiler bug (file ordering / dependency, perhaps?).

Sources/NonEmpty/NonEmpty+Collection.swift

@@ -1,4 +1,4 @@
-extension NonEmpty: Collection {
+extension NonEmpty {
   public enum Index: Comparable {
     case head
     case tail(C.Index)

Sources/NonEmpty/NonEmpty+MutableCollection.swift

@@ -28,7 +28,7 @@ extension NonEmpty: MutableCollection where C: MutableCollection {
 }
 
 extension NonEmpty where C: MutableCollection, C.Index == Int {
-  public subscript(position: Int) -> Element {
+  public subscript(position: C.Index) -> Element {
     get {
       return self[position == 0 ? .head : .tail(self.tail.startIndex + position - 1)]
     }

Sources/NonEmpty/NonEmpty.swift

@@ -1,4 +1,4 @@
-public struct NonEmpty<C: Collection> {
+public struct NonEmpty<C: Collection>: Collection {
   public typealias Element = C.Element
 
   public internal(set) var head: Element