Cleanup fmt.hh

When I started contributing to Nix, I found the mix of definitions and
names in `fmt.hh` to be rather confusing, especially the small
difference between `hintfmt` and `hintformat`. I've renamed many classes
and added documentation to most definitions.

- `formatHelper` is no longer exported.
- `fmt`'s documentation is now with `fmt` rather than (misleadingly)
  above `formatHelper`.
- `yellowtxt` is renamed to `Magenta`.

  `yellowtxt` wraps its value with `ANSI_WARNING`, but `ANSI_WARNING`
  has been equal to `ANSI_MAGENTA` for a long time. Now the name is
  updated.
- `normaltxt` is renamed to `Uncolored`.
- `hintfmt` has been merged into `hintformat` as extra constructor
  functions.
- `hintformat` has been renamed to `hintfmt`.
- The single-argument `hintformat(std::string)` constructor has been
  renamed to a static member `hintformat::interpolate` to avoid pitfalls
  with using user-generated strings as format strings.

src/libutil/error.cc

@@ -11,7 +11,7 @@
 
 namespace nix {
 
-void BaseError::addTrace(std::shared_ptr<Pos> && e, hintformat hint, bool frame)
+void BaseError::addTrace(std::shared_ptr<Pos> && e, hintfmt hint, bool frame)
 {
     err.traces.push_front(Trace { .pos = std::move(e), .hint = hint, .frame = frame });
 }
@@ -37,7 +37,7 @@ const std::string & BaseError::calcWhat() const
 
 std::optional<std::string> ErrorInfo::programName = std::nullopt;
 
-std::ostream & operator <<(std::ostream & os, const hintformat & hf)
+std::ostream & operator <<(std::ostream & os, const hintfmt & hf)
 {
     return os << hf.str();
 }