string literals should not allocate; should allocate in ROM an extra `\0`

[timothee]

in D, string literals don't allocate. (in C as well but they decay to pointers 
so let's leave these aside from this discussion)

D: 
    
    
    // D20180718T163602
    import std.stdio;
    
    extern(C) void fun(const(char)* str){
      printf("std:{%s}\n", str);
    }
    
    void main(){
      string a = "foo";
      // a.ptr points to ROM to a block of size 3+1 (last entry is 0, to help 
implicit conversion without allocation to C strings)
      assert(a.length == 3);
      assert(a.ptr[3] == 0);
      fun(a.ptr);
      
      auto a2 = new char[](a.length);
      a2[]=a;
      assert(a.ptr[3] == 0); // probably fails
      fun(a2.ptr); // undefined behavior, since it's not null terminated
    }
    
    
    Run

Seems like Nim could use these 2 tricks as well:

  * avoiding allocation for var a = "foo" * allocate in ROM an extra byte to 
hold `0 for null termination