Add & demo wrapTypecheckPushUntyped wrapper

Before this wrapper, the example in the README was harder to write:

```
*PTS.Interactive> Right a <- wrapTypecheckPull (Just Instances.uu) [pts|* -> *|] []
*PTS.Interactive> wrapTypecheckPush (Just Instances.uu) [pts|lambda x. x|] a []
```

I also find typecheckPushUntyped's implementation rather elegant.