update macros.add()

have a test for it if you want it:
```nimrod
discard """
1
2
3"""
import macros

macro foo: stmt =
  result = newNimNode(nnkStmtList).add(
    newCall("echo", newIntLitNode(1)),
    newCall("echo", newIntLitNode(2)))
  result.add newCall("echo", newIntLitNode(3))

foo
```