Make the proc with iterator parameter overloadable

[slangmgh]

If there is a macro accept an iterator parameter, it will no compile if there is same name proc with other typed parameter, that's very annoying, toSeq has this problem.

Compile the following code will generate error.

import macros

# comment this proc will make the compiler works
proc p1(x: int) =  echo x

macro p1(s: untyped): untyped =
   quote do:
      for n in `s`:
         echo n

iterator it(): int =
   yield 1; yield 2

p1(it())

Compiler output:

D:\Work\Nim\dd.nim(14, 6) Error: attempting to call routine: 'it'
  found 'dd.it() [declared in D:\Work\Stock\dd.nim(11, 10)]' of kind 'iterator'