line cap and dimensions bindings

So, if you `show` something, you should be able to take the string and paste it into the REPL, and it should work, but `round`, for example does not evaluate to `Round`. This should be

```
show Round = "Round"
```

etc. with another function used to turn `LineCap`s into `String`s for passing to the `setLineCapImpl` function.

Either that or make `LineCap` into a `newtype` around `String`.