If strings are immutable in L0x then this string representation:

    struct sObjString {
      Obj obj;         
      int length;      
      char* chars;     
    };   

needlessly wastes 4 or 8 bytes for the chars pointer (among other heap management overhead related to separate allocations).

Instead, I think, you should have

    struct sObjString {
      Obj  obj;         
      int  length;      
      char chars[0];     
    };   

and allocate that string object as one chunk with its characters.