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/libexpr/eval.hh

@@ -148,7 +148,7 @@ struct DebugTrace {
     std::shared_ptr<Pos> pos;
     const Expr & expr;
     const Env & env;
-    hintformat hint;
+    hintfmt hint;
     bool isError;
 };
 

src/libexpr/value/context.hh

@@ -20,7 +20,7 @@ public:
     {
         raw = raw_;
         auto hf = hintfmt(args...);
-        err.msg = hintfmt("Bad String Context element: %1%: %2%", normaltxt(hf.str()), raw);
+        err.msg = hintfmt("Bad String Context element: %1%: %2%", Uncolored(hf.str()), raw);
     }
 };