make more standard libraries work with nimPreviewSlimSystem (#20343)

* make more standard libraries work with `nimPreviewSlimSystem`

* typo

* part two

* Delete specutils.nim

* fixes more tests

* more fixes

* fixes tests

* fixes three more tests

* add formatfloat import

* fix

* last

lib/pure/concurrency/cpuload.nim

@@ -19,6 +19,9 @@ when defined(windows):
 elif defined(linux):
   from cpuinfo import countProcessors
 
+when defined(nimPreviewSlimSystem):
+  import std/syncio
+
 type
   ThreadPoolAdvice* = enum
     doNothing,

lib/pure/concurrency/threadpool.nim

@@ -23,6 +23,9 @@ when not compileOption("threads"):
 
 import cpuinfo, cpuload, locks, os
 
+when defined(nimPreviewSlimSystem):
+  import std/assertions
+
 {.push stackTrace:off.}
 
 type