bootstrap the compiler with nimPreviewSlimSystem (#20176)

* bootstrap the compiler with nimPreviewSlimSystem

* threads

lib/pure/collections/lists.nim

@@ -59,6 +59,9 @@ runnableExamples:
 
 import std/private/since
 
+when defined(nimPreviewSlimSystem):
+  import std/assertions
+
 when not defined(nimHasCursor):
   {.pragma: cursor.}
 

lib/pure/strscans.nim

@@ -286,6 +286,10 @@ efficiency and perform different checks.
 import macros, parseutils
 import std/private/since
 
+when defined(nimPreviewSlimSystem):
+  import std/assertions
+
+
 proc conditionsToIfChain(n, idx, res: NimNode; start: int): NimNode =
   assert n.kind == nnkStmtList
   if start >= n.len: return newAssignment(res, newLit true)

lib/pure/uri.nim

@@ -39,6 +39,9 @@ runnableExamples:
 import strutils, parseutils, base64
 import std/private/[since, decode_helpers]
 
+when defined(nimPreviewSlimSystem):
+  import std/assertions
+
 
 type
   Url* = distinct string

lib/pure/xmltree.nim

@@ -33,6 +33,10 @@ runnableExamples:
 import std/private/since
 import macros, strtabs, strutils
 
+when defined(nimPreviewSlimSystem):
+  import std/assertions
+
+
 type
   XmlNode* = ref XmlNodeObj ## An XML tree consisting of XML nodes.
     ##