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.
While the discussion about how to refactor test framework is not
finished (#1654), make this change: move `rm -rf build` in the
beginning of the test. For these reasons:
* it is useful to inspect the contents of the `build` directory
especially after the test failure
* if build crashes mid-test (e.g. process killed), next run should
not be affected by the `build` directory from the previous run
