Support Tuple<T1, T2> syntax by default

It's often useful to specify values of type `Tuple<T1, T2>` as a two-element list.  e.g.

```
position: [10, 15]
```
should be parse-able with the following
```
System.Tuple<float,float> position = Doc["position"].As<System.Tuple<float,float>>();
```

There should be a case for handling generic tuple types in `TypeReifier`.