builtin: fix v -W -Wimpure-v examples/hello_world.v (move ctovstring_impl to builtin.c.v)

Author: spytheman

cmd/tools/vtest-all.v

@@ -90,6 +90,11 @@ fn get_all_commands() []Command {
 		okmsg:  'V can compile hello world.'
 		rmfile: 'examples/hello_world'
 	}
+	res << Command{
+		line:   '${vexe} -W -Wimpure-v run examples/hello_world.v'
+		okmsg:  'V can compile hello world with the stricter `-W -Wimpure-v` mode .'
+		rmfile: 'examples/hello_world'
+	}
 	$if linux {
 		if l2w_crosscc != '' {
 			res << Command{

vlib/builtin/builtin.c.v

@@ -944,3 +944,16 @@ pub fn arguments() []string {
 	}
 	return res
 }
+
+// ctovstring_impl is a temporary API, to enable clean CI runs for https://github.com/vlang/v/pull/25264 .
+// It will be deleted after the migration to the new `builtin` naming scheme is finished.
+@[export: 'builtin__ctovstring']
+pub fn ctovstring_impl(s &u8) string {
+	unsafe {
+		len := C.strlen(voidptr(s))
+		return string{
+			str: memdup(voidptr(s), isize(len))
+			len: len
+		}
+	}
+}

vlib/builtin/string.v

@@ -3052,16 +3052,3 @@ pub fn (mut ri RunesIterator) next() ?rune {
 	}
 	return rune(impl_utf8_to_utf32(start, len))
 }
-
-// ctovstring_impl is a temporary API, to enable clean CI runs for https://github.com/vlang/v/pull/25264 .
-// It will be deleted after the migration to the new `builtin` naming scheme is finished.
-@[export: 'builtin__ctovstring']
-pub fn ctovstring_impl(s &u8) string {
-	unsafe {
-		len := C.strlen(voidptr(s))
-		return string{
-			str: memdup(voidptr(s), isize(len))
-			len: len
-		}
-	}
-}