Array.removeAt

Hi,

Deepnight and I think haxe Array misses :

```
function removeAt( pos : Int) : Void;
```

when there is only :

```
function splice( pos : Int, len : Int ) : Array;
```

Splice is very problematique since int creates another array and totally trashes memory. On most platforms this function already exists so here is a big abstraction leak that haxe should fill.

for example on cpp, ths function would not ungrow the memory zone but use a memmove to retrieve the overlappig pointer zone, thus providing a very fast implementation.

So we propose we introduce this function in cross platform.

we can start with a naive implementation which is

```
function removeAt( pos : Int) : Void {
arr.remove( arr[idx] );
}
```

For informations, array is very often the (very) worst offender in memory usage yet the best ergonomic alternative, so we think improving it would be great.