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/libstore/sqlite.cc

@@ -10,19 +10,19 @@
 
 namespace nix {
 
-SQLiteError::SQLiteError(const char *path, const char *errMsg, int errNo, int extendedErrNo, int offset, hintformat && hf)
+SQLiteError::SQLiteError(const char *path, const char *errMsg, int errNo, int extendedErrNo, int offset, hintfmt && hf)
   : Error(""), path(path), errMsg(errMsg), errNo(errNo), extendedErrNo(extendedErrNo), offset(offset)
 {
     auto offsetStr = (offset == -1) ? "" : "at offset " + std::to_string(offset) + ": ";
     err.msg = hintfmt("%s: %s%s, %s (in '%s')",
-        normaltxt(hf.str()),
+        Uncolored(hf.str()),
         offsetStr,
         sqlite3_errstr(extendedErrNo),
         errMsg,
         path ? path : "(in-memory)");
 }
 
-[[noreturn]] void SQLiteError::throw_(sqlite3 * db, hintformat && hf)
+[[noreturn]] void SQLiteError::throw_(sqlite3 * db, hintfmt && hf)
 {
     int err = sqlite3_errcode(db);
     int exterr = sqlite3_extended_errcode(db);