Merge indexed_set.rs into bitvec.rs, and rename it bit_set.rs.

Currently we have two files implementing bitsets (and 2D bit matrices).
This commit combines them into one, taking the best features from each.

This involves renaming a lot of things. The high level changes are as
follows.
- bitvec.rs              --> bit_set.rs
- indexed_set.rs         --> (removed)
- BitArray + IdxSet      --> BitSet (merged, see below)
- BitVector              --> GrowableBitSet
- {,Sparse,Hybrid}IdxSet --> {,Sparse,Hybrid}BitSet
- BitMatrix              --> BitMatrix
- SparseBitMatrix        --> SparseBitMatrix

The changes within the bitset types themselves are as follows.

```
OLD             OLD             NEW
BitArray<C>     IdxSet<T>       BitSet<T>
--------        ------          ------
grow            -               grow
new             -               (remove)
new_empty       new_empty       new_empty
new_filled      new_filled      new_filled
-               to_hybrid       to_hybrid
clear           clear           clear
set_up_to       set_up_to       set_up_to
clear_above     -               clear_above
count           -               count
contains(T)     contains(&T)    contains(T)
contains_all    -               superset
is_empty        -               is_empty
insert(T)       add(&T)         insert(T)
insert_all      -               insert_all()
remove(T)       remove(&T)      remove(T)
words           words           words
words_mut       words_mut       words_mut
-               overwrite       overwrite
merge           union           union
-               subtract        subtract
-               intersect       intersect
iter            iter            iter
```

In general, when choosing names I went with:
- names that are more obvious (e.g. `BitSet` over `IdxSet`).
- names that are more like the Rust libraries (e.g. `T` over `C`,
  `insert` over `add`);
- names that are more set-like (e.g. `union` over `merge`, `superset`
  over `contains_all`, `domain_size` over `num_bits`).

Also, using `T` for index arguments seems more sensible than `&T` --
even though the latter is standard in Rust collection types -- because
indices are always copyable. It also results in fewer `&` and `*`
sigils in practice.

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