MUST allow to rewrite function that has the same signature in the interactive mode

[kkuramitsu]

To improve the usability of interactive mode..

Konoha 3.0 (Rome) (92081f66, 66205, Mar 29 2013)
[gcc 4.2.1 Compatible Apple LLVM 4.2 (clang-425.0.24)]
modules: MiniVM bmgengc iconv
>>> 1.0
>>> 
>>> int f(int n) {
      return 1;
    }
>>> f(1)
  (int) 1
>>> int f(int n) {  
  return n+1;       
 }
 - (error) ((shell):6) duplicated method: NameSpace.f on konoha