fixes ptr to cstring warnings[backport] (#20848)

* fix =#13790 ptr char (+friends) should not implicitly convert to cstring

* Apply suggestions from code review

* first round; compiles on windows

* nimPreviewSlimSystem

* conversion is unsafe, cast needed

* fixes more tests

* fixes asyncnet

* another try another error

* last one

* true

* one more

* why bugs didn't show at once

* add `nimPreviewCstringConversion` switch

* typo

* fixes ptr to cstring warnings[backport]

* add fixes

Co-authored-by: xflywind <43030857+xflywind@users.noreply.github.com>

tests/system/tostring.nim

@@ -47,7 +47,7 @@ import strutils
 
 let arr = ['H','e','l','l','o',' ','W','o','r','l','d','!','\0']
 doAssert $arr == "['H', 'e', 'l', 'l', 'o', ' ', 'W', 'o', 'r', 'l', 'd', '!', '\\x00']"
-doAssert $cstring(addr arr) == "Hello World!"
+doAssert $cast[cstring](addr arr) == "Hello World!"
 
 proc takes(c: cstring) =
   doAssert c == cstring""