Make to_str pure and fix const parameters for str-mutating functions

Two separate changes that got intertwined (sorry):

Make to_str pure. Closes #3691

In str, change functions like push_char to take an &mut str instead of
an &str. Closes #3710

src/test/run-pass/class-separate-impl.rs

@@ -45,7 +45,7 @@ fn cat(in_x : uint, in_y : int, in_name: ~str) -> cat {
 }
 
 impl cat: ToStr {
-  fn to_str() -> ~str { self.name }
+  pure fn to_str() -> ~str { self.name }
 }
 
 fn print_out<T: ToStr>(thing: T, expected: ~str) {