Add a Repr trait that's distinct from ToStr (or a repr() method on ToStr)

[jesse99]

This simple unit test fails:

#[test]
fn test_int()
{
   let x = ~10;
   assert x.to_str() == ~"10";
}

because the returned string is actually ~"~10". That's rather surprising.

Same sort of thing occurs with path::Path.

[pcwalton]

Yeah, we should have ToStr distinct from Repr, the latter of which returns a more low-level representation, and the former of which returns a string suitable for displaying to users.

[graydon]

this is fixed, repr is a separate trait now