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

tests/stdlib/tsequtils.nim

@@ -7,6 +7,7 @@ discard """
 import std/sequtils
 import strutils
 from algorithm import sorted
+import std/assertions
 
 {.experimental: "strictEffects".}
 {.push warningAsError[Effect]: on.}