move assertions out of system (#19599)

lib/pure/collections/hashcommon.nim

@@ -10,6 +10,10 @@
 # An `include` file which contains common code for
 # hash sets and tables.
 
+when defined(nimPreviewSlimSystem):
+  import std/assertions
+
+
 const
   growthFactor = 2
 

lib/pure/collections/sequtils.nim

@@ -84,6 +84,10 @@ import std/private/since
 
 import macros
 
+when defined(nimPreviewSlimSystem):
+  import std/assertions
+
+
 when defined(nimHasEffectsOf):
   {.experimental: "strictEffects".}
 else: