Fix: don't lose monotonic clock precision on darwin

src/crystal/system/unix/time.cr

@@ -49,9 +49,10 @@ module Crystal::System::Time
 
   def self.monotonic
     {% if flag?(:darwin) %}
-      info = mach_timebase_info
-      nanoseconds = LibC.mach_absolute_time.to_i64 * info.numer / info.denom
-      {nanoseconds / 1_000_000_000, nanoseconds.remainder(1_000_000_000).to_i32}
+      absolute_time = LibC.mach_absolute_time * mach_ticks_per_nanosecond
+      seconds = absolute_time / 1_000_000_000
+      nanoseconds = absolute_time.remainder(1_000_000_000)
+      {seconds.to_i64, nanoseconds.to_i32}
     {% else %}
       if LibC.clock_gettime(LibC::CLOCK_MONOTONIC, out tp) == 1
         raise Errno.new("clock_gettime(CLOCK_MONOTONIC)")
@@ -61,12 +62,12 @@ module Crystal::System::Time
   end
 
   {% if flag?(:darwin) %}
-    @@mach_timebase_info : LibC::MachTimebaseInfo?
+    @@mach_ticks_per_nanosecond : Float64?
 
-    private def self.mach_timebase_info
-      @@mach_timebase_info ||= begin
+    private def self.mach_ticks_per_nanosecond
+      @@mach_ticks_per_nanosecond ||= begin
         LibC.mach_timebase_info(out info)
-        info
+        info.numer.to_f64 / info.denom
       end
     end
   {% end %}