Fix current-stage compiler stats output

Before be945b5, --stats output would print the current stage name while it was
executing. This is useful for debugging compiler hangs and tracking the progress
of the compiler without --progress. After that commit, the progress output would
only print after the stage had fully completed, meaning you had to rely on
--progress to track compiler progress.

This patch fixes this regression by printing the stage name (with no times or
memory figures) before the stage starts and resetting the cursor to the
beginning of the line. If both --progress and --stats are specified, this
pre-stage line isn't printed as it would just be overwritten by --progress
output anyway.

src/compiler/crystal/progress_tracker.cr

@@ -15,6 +15,7 @@ module Crystal
     def stage(name)
       @current_stage_name = name
 
+      print_stats
       print_progress
 
       start_time = Time.now
@@ -37,13 +38,17 @@ module Crystal
       print "\r"
     end
 
-    def print_stats(time_taken)
+    def print_stats(time_taken = nil)
       return unless @stats
 
-      memory_usage_mb = GC.stats.heap_size / 1024.0 / 1024.0
-      memory_usage_str = " (%7.2fMB)" % {memory_usage_mb} if true # display_memory?
       justified_name = "#{current_stage_name}:".ljust(STAGE_PADDING)
-      puts "#{justified_name} #{time_taken}#{memory_usage_str}"
+      if time_taken
+        memory_usage_mb = GC.stats.heap_size / 1024.0 / 1024.0
+        memory_usage_str = " (%7.2fMB)" % {memory_usage_mb} if true # display_memory?
+        puts "#{justified_name} #{time_taken}#{memory_usage_str}"
+      else
+        print "#{justified_name}\r" unless @progress
+      end
     end
 
     def print_progress