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/build/derivation-goal.cc

@@ -708,7 +708,7 @@ void DerivationGoal::tryToBuild()
     if (!outputLocks.lockPaths(lockFiles, "", false)) {
         if (!actLock)
             actLock = std::make_unique<Activity>(*logger, lvlWarn, actBuildWaiting,
-                fmt("waiting for lock on %s", magentatxt(showPaths(lockFiles))));
+                fmt("waiting for lock on %s", Magenta(showPaths(lockFiles))));
         worker.waitForAWhile(shared_from_this());
         return;
     }
@@ -762,7 +762,7 @@ void DerivationGoal::tryToBuild()
                    the wake-up timeout expires. */
                 if (!actLock)
                     actLock = std::make_unique<Activity>(*logger, lvlWarn, actBuildWaiting,
-                        fmt("waiting for a machine to build '%s'", magentatxt(worker.store.printStorePath(drvPath))));
+                        fmt("waiting for a machine to build '%s'", Magenta(worker.store.printStorePath(drvPath))));
                 worker.waitForAWhile(shared_from_this());
                 outputLocks.unlock();
                 return;
@@ -987,7 +987,7 @@ void DerivationGoal::buildDone()
             diskFull |= cleanupDecideWhetherDiskFull();
 
             auto msg = fmt("builder for '%s' %s",
-                magentatxt(worker.store.printStorePath(drvPath)),
+                Magenta(worker.store.printStorePath(drvPath)),
                 statusToString(status));
 
             if (!logger->isVerbose() && !logTail.empty()) {
@@ -1523,7 +1523,7 @@ void DerivationGoal::done(
     outputLocks.unlock();
     buildResult.status = status;
     if (ex)
-        buildResult.errorMsg = fmt("%s", normaltxt(ex->info().msg));
+        buildResult.errorMsg = fmt("%s", Uncolored(ex->info().msg));
     if (buildResult.status == BuildResult::TimedOut)
         worker.timedOut = true;
     if (buildResult.status == BuildResult::PermanentFailure)

src/libstore/build/local-derivation-goal.cc

@@ -232,7 +232,7 @@ void LocalDerivationGoal::tryLocalBuild()
         if (!buildUser) {
             if (!actLock)
                 actLock = std::make_unique<Activity>(*logger, lvlWarn, actBuildWaiting,
-                    fmt("waiting for a free build user ID for '%s'", magentatxt(worker.store.printStorePath(drvPath))));
+                    fmt("waiting for a free build user ID for '%s'", Magenta(worker.store.printStorePath(drvPath))));
             worker.waitForAWhile(shared_from_this());
             return;
         }