Refactor ty_str to use a ~(str) representation.

Similar to my recent changes to ~[T]/&[T], these changes remove the vstore abstraction and represent str types as ~(str) and &(str). The Option<uint> in ty_str is the length of the string, None if the string is dynamically sized.

src/test/compile-fail/estr-subtyping.rs

@@ -17,7 +17,7 @@ fn has_uniq(x: ~str) {
 }
 
 fn has_slice(x: &str) {
-   wants_uniq(x); //~ ERROR str storage differs: expected `~` but found `&`
+   wants_uniq(x); //~ ERROR mismatched types: expected `~str` but found `&str` (expected ~-ptr but f
    wants_slice(x);
 }
 

src/test/compile-fail/ifmt-bad-select.rs

@@ -10,5 +10,5 @@
 
 fn main() {
     format!("{0, select, other{}}", 2);
-    //~^ ERROR: expected &str but found integral
+    //~^ ERROR: mismatched types: expected `&&str` but found `&<generic integer #0>` (expected &-ptr
 }