Fix: prefer System::Time.monotonic over Time.monotonic

Avoids an issue with the timecop shard that overrides Time.monotonic.

src/crystal/system/unix/evented/event.cr

@@ -34,7 +34,11 @@ struct Crystal::Evented::Event
   include PointerLinkedList::Node
 
   def initialize(@type : Type, @fiber, @index = nil, timeout : Time::Span? = nil)
-    @wake_at = Time.monotonic + timeout if timeout
+    if timeout
+      seconds, nanoseconds = System::Time.monotonic
+      now = Time::Span.new(seconds: seconds, nanoseconds: nanoseconds)
+      @wake_at = now + timeout
+    end
   end
 
   # Mark the IO event as timed out.

src/crystal/system/unix/evented/fiber_event.cr

@@ -7,7 +7,9 @@ class Crystal::Evented::FiberEvent
 
   # sleep or select timeout
   def add(timeout : Time::Span) : Nil
-    @event.wake_at = Time.monotonic + timeout
+    seconds, nanoseconds = System::Time.monotonic
+    now = Time::Span.new(seconds: seconds, nanoseconds: nanoseconds)
+    @event.wake_at = now + timeout
     @event_loop.add_timer(pointerof(@event))
   end
 

src/crystal/system/unix/evented/timers.cr

@@ -29,11 +29,13 @@ struct Crystal::Evented::Timers
   end
 
   # Dequeues and yields each ready timer (their `#wake_at` is lower than
-  # `Time.monotonic`) from the oldest to the most recent (i.e. time ascending).
+  # `System::Time.monotonic`) from the oldest to the most recent (i.e. time
+  # ascending).
   def dequeue_ready(& : Evented::Event* -> Nil) : Nil
     return if @list.empty?
 
-    now = Time.monotonic
+    seconds, nanoseconds = System::Time.monotonic
+    now = Time::Span.new(seconds: seconds, nanoseconds: nanoseconds)
     n = 0
 
     @list.each do |event|

src/crystal/system/unix/kqueue/event_loop.cr

@@ -214,7 +214,11 @@ class Crystal::Kqueue::EventLoop < Crystal::Evented::EventLoop
   private def system_set_timer(time : Time::Span?) : Nil
     if time
       flags = LibC::EV_ADD | LibC::EV_ONESHOT | LibC::EV_CLEAR
-      t = time - Time.monotonic
+
+      seconds, nanoseconds = System::Time.monotonic
+      now = Time::Span.new(seconds: seconds, nanoseconds: nanoseconds)
+      t = time - now
+
       data =
         {% if LibC.has_constant?(:NOTE_NSECONDS) %}
           t.total_nanoseconds.to_i64!.clamp(0..)