Instead of having UN & RF (& Hole in the near future & maybe even
more later e.g. operator names) we have a single UN constructor
that takes a UserName instead of a String.
UserName is (for now)
```idris
data UserName : Type where
Basic : String -> UserName -- default name constructor e.g. map
Field : String -> UserName -- field accessor e.g. .fst
Underscore : UserName -- no name e.g. _
```
This is extracted from the draft PR #1852 which is too big to easily
debug. Once this is working, I can go back to it.
