Improve handling of type bounds in bit_set.rs.

Currently, `BitSet` doesn't actually know its own domain size; it just
knows how many words it contains. To improve things, this commit makes
the following changes.

- It changes `BitSet` and `SparseBitSet` to store their own domain size,
  and do more precise bounds and same-size checks with it. It also
  changes the signature of `BitSet::to_string()` (and puts it within
  `impl ToString`) now that the domain size need not be passed in from
  outside.

- It uses `derive(RustcDecodable, RustcEncodable)` for `BitSet`. This
  required adding code to handle `PhantomData` in `libserialize`.

- As a result, it removes the domain size from `HybridBitSet`, making a
  lot of that code nicer.

- Both set_up_to() and clear_above() were overly general, working with
  arbitrary sizes when they are only needed for the domain size. The
  commit removes the former, degeneralizes the latter, and removes the
  (overly general) tests.

- Changes `GrowableBitSet::grow()` to `ensure()`, fixing a bug where a
  (1-based) domain size was confused with a (0-based) element index.

- Changes `BitMatrix` to store its row count, and do more precise bounds
  checks with it.

- Changes `ty_params` in `select.rs` from a `BitSet` to a
  `GrowableBitSet` because it repeatedly failed the new, more precise
  bounds checks. (Changing the type was simpler than computing an
  accurate domain size.)

- Various other minor improvements.

src/librustc/traits/select.rs

@@ -44,7 +44,7 @@ use ty::relate::TypeRelation;
 use middle::lang_items;
 use mir::interpret::{GlobalId};
 
-use rustc_data_structures::bit_set::BitSet;
+use rustc_data_structures::bit_set::GrowableBitSet;
 use rustc_data_structures::sync::Lock;
 use std::iter;
 use std::cmp;
@@ -3069,7 +3069,7 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
                 } else {
                     return Err(Unimplemented);
                 };
-                let mut ty_params = BitSet::new_empty(substs_a.types().count());
+                let mut ty_params = GrowableBitSet::new_empty();
                 let mut found = false;
                 for ty in field.walk() {
                     if let ty::Param(p) = ty.sty {