Refactor representation of bounds to separate out BuiltinBounds into

its own type. Use a bitset to represent built-in bounds. There
are several places in the language where only builtin bounds (aka kinds)
will be accepted, e.g. on closures, destructor type parameters perhaps,
and on trait types.

src/test/compile-fail/issue-2611-4.rs

@@ -20,7 +20,7 @@ struct E {
 }
 
 impl A for E {
-  fn b<F:Copy + Const,G>(_x: F) -> F { fail!() } //~ ERROR in method `b`, type parameter 0 has 2 bounds, but
+  fn b<F:Copy + Const,G>(_x: F) -> F { fail!() } //~ ERROR type parameter 0 requires `Const`
 }
 
 fn main() {}

src/test/run-pass/issue-2611-3.rs

@@ -8,11 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// Tests that impl methods are matched to traits exactly:
-// we might be tempted to think matching is contravariant, but if
-// we let an impl method can have more permissive bounds than the trait
-// method it's implementing, the return type might be less specific than
-// needed. Just punt and make it invariant.
+// Tests that impls are allowed to have looser, more permissive bounds
+// than the traits require.
 
 trait A {
   fn b<C:Copy + Const,D>(x: C) -> C;