Mark system.newStringUninit sideeffect-free (#24813)

- Allows using with `--experimental:strictFuncs`
- `{.cast(noSideEffect).}:` inside the proc was required to mutate
`s.len`, same as used in `newSeqImpl`.
- Removed now unnecessary `noSideEffect` casts in `system.nim`
-
Closes #24811

Co-authored-by: ringabout <43030857+ringabout@users.noreply.github.com>
(cherry picked from commit ecdcffed4b)

lib/pure/strutils.nim

@@ -2356,8 +2356,7 @@ func multiReplace*(s: openArray[char]; replacements: varargs[(set[char], char)])
     # Sanitize a filename with Windows-incompatible characters
     const file = "a/file:with?invalid*chars.txt"
     doAssert file.multiReplace(WinSanitationRules) == "a-file-with_invalid_chars.txt"
-  {.cast(noSideEffect).}:
-    result = newStringUninit(s.len)
+  result = newStringUninit(s.len)
   for i in 0..<s.len:
     var nextChar = s[i]
     for subs, by in replacements.items: