string: use correct allocator in replace (#2405)

`replace` accepts an Allocator but uses `mem` instead.

Author: konimarti

lib/std/core/string.c3

@@ -187,7 +187,7 @@ fn String String.replace(self, Allocator allocator, String needle, String new_st
 	@pool()
 	{
 		String[] split = self.tsplit(needle);
-		return dstring::join(tmem, split, new_str).copy_str(mem);
+		return dstring::join(tmem, split, new_str).copy_str(allocator);
 	};
 }
 

releasenotes.md

@@ -39,6 +39,7 @@
 - Slicing a constant array with designated initialization would not update the indexes.
 - Fix for bug when `@format` encountered `*` in some cases.
 - Compiler segfault on global slice initialization with null[:0] #2404.
+- Use correct allocator in `replace`.
 
 ### Stdlib changes
 - Add `==` to `Pair`, `Triple` and TzDateTime. Add print to `Pair` and `Triple`.