[WIP} Added a WIP array.mean() implementation.

Related to issue #828 and some others.
No, there's no sophistication. The decorators ("builtin" and "implement") don't actually create a new function, they just return the original one. Which means that the solution is even simpler than what I suggested. The following should work:

```
@builtin
@implement(numpy.var, types.Kind(types.Array))
@implement("array.var", types.Kind(types.Array))
def [etc.]
```