fix some warnings (#16952)

lib/pure/streamwrapper.nim

@@ -31,7 +31,7 @@ proc posReadLine[T](s: Stream, line: var string): bool =
   assert s.baseReadLineImpl != nil
 
   let n = s.buffer.len
-  line.string.setLen(0)
+  line.setLen(0)
   for i in 0..<n:
     var c = s.buffer.popFirst
     if c == '\c':
@@ -40,7 +40,7 @@ proc posReadLine[T](s: Stream, line: var string): bool =
     elif c == '\L': return true
     elif c == '\0':
       return line.len > 0
-    line.string.add(c)
+    line.add(c)
 
   var line2: string
   result = s.baseReadLineImpl(s, line2)