Show changes

I've been debugging Idris a bit to figure out how it works and I've had some problems with the `Show` instances:

* `Show Name` was used for opening files (i.e. changing the implementation would crash Idris)
  * Seems like using `showCG` makes more sense
* `Show PDecl` was partial (would crash on certain nodes)
  * Seems like the implementation of the total `dumpDecl` function was superior
* `showCG` would call `show` instead of being recursive

I'm happy to remove any changes from this PR if they don't make sense.