Fix(SlimSystem): use import instead of include

Author: noahehall

threading/channels.nim

@@ -83,8 +83,8 @@ when not defined(gcArc) and not defined(gcOrc) and not defined(nimdoc):
   {.error: "This channel implementation requires --gc:arc or --gc:orc".}
 
 when defined(nimPreviewSlimSystem):
-  include std/typedthreads
-  include std/assertions
+  import std/typedthreads
+  import std/assertions
 
 import std/[locks, atomics, isolation]
 import system/ansi_c