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.

tests/unit/libexpr/error_traces.cc

@@ -53,7 +53,6 @@ namespace nix {
                 state.error<EvalError>("beans").debugThrow();
             } catch (Error & e2) {
                 e.addTrace(state.positions[noPos], "beans2", "");
-                //e2.addTrace(state.positions[noPos], "Something", "");
                 ASSERT_TRUE(e.info().traces.size() == 2);
                 ASSERT_TRUE(e2.info().traces.size() == 0);
                 ASSERT_FALSE(&e.info() == &e2.info());