collections::bitv: ensure correct masking behaviour

The internal masking behaviour for `Bitv` is now defined as:
  - Any entirely words in self.storage must be all zeroes.
  - Any partially used words may have anything at all in their
    unused bits.

This means:
  - When decreasing self.nbits, care must be taken that any
    no-longer-used words are zeroed out.

  - When increasing self.nbits, care must be taken that any
    newly-unmasked bits are set to their correct values.

  - When reading words, care should be taken that the values of
    unused bits are not used. (Preferably, use `Bitv::mask_words`
    which zeroes them out for you.)

The old behaviour was that every unused bit was always set to
zero. The problem with this is that unused bits are almost never
read, so forgetting to do this will result in very subtle and
hard-to-track down bugs. This way the responsibility for masking
falls on the places which might cause unused bits to be read: for
now, this is only `Bitv::mask_words` and `BitvSet::insert`.
collections::bitv: correct use of Vec<T>::grow

The argument passed to Vec::grow is the number of elements to grow
the vector by, not the target number of elements. The old `Bitv`
code did the wrong thing, allocating more memory than it needed to.
collections::bitv: remove some ancient interfaces

Removes the following methods from `Bitv`:

  - `to_vec`: translates a `Bitv` into a bulky `Vec<uint>` of 0's and 1's
    replace with:  `bitv.iter().map(|b| if b {1} else {0}).collect()`

  - `to_bools`: translates a `Bitv` into a `Vec<bool>`
    replace with: `bitv.iter().collect()`

  - `ones`: internal iterator over all 1 bits in a `Bitv`
    replace with: `BitvSet::from_bitv(bitv).iter().advance(fn)`

These methods had specific functionality which can be replicated more
generally by the modern iterator system. (Also `to_vec` was not even
unit tested!)
collections::bitv: replace internal iterators with external ones
collections::bitv: Add documentation and #[inline]'s

Add documentation to methods on BitvSet that were missing them. Also
make sure #[inline] is on all methods that are (a) one-liners or (b)
private methods whose only purpose is code deduplication.
collections::bitv: Implement several methods for `Bitv` and `BitvSet`

On Bitv:
   - Add .push() and .pop() which take and return bool, respectively
   - Add .truncate() which truncates a Bitv to a specific length
   - Add .grow() which grows a Bitv by a specific length
   - Add .reserve() which grows the underlying storage to be able to hold
     a specified number of bits without resizing
   - Implement FromIterator<Vec<bool>>
   - Implement Extendable<bool>
   - Implement Collection
   - Implement Mutable
   - Remove .from_bools() since FromIterator<Vec<bool>> now accomplishes this.
   - Remove .assign() since Clone::clone_from() accomplishes this.

On BitvSet:
   - Add .reserve() which grows the underlying storage to be able to hold
     a specified number of bits without resizing
   - Add .get_ref() and .get_mut_ref() to return references to the
     underlying Bitv
collections::bitv: change constructors for `Bitv` and `BitvSet`

`Bitv::new` has been renamed `Bitv::with_capacity`. The new function
`Bitv::new` now creates a `Bitv` with no elements.

The new function `BitvSet::with_capacity` creates a `BitvSet` with
a specified capacity.
collections::bitv: clean up and unit test `is_subset`