collections::bitv: Remove SmallBitv/BigBitv dichotomy

The old `Bitv` structure had two variations: one represented by a vector of
uints, and another represented by a single uint for bit vectors containing
fewer than uint::BITS bits.

The purpose of this is to avoid the indirection of using a Vec, but the
speedup is only available to users who

  (a) are storing less than uints::BITS bits
  (b) know this when they create the vector (since `Bitv`s cannot be resized)
  (c) don't know this at compile time (else they could use uint directly)

Giving such specific users a (questionable) speed benefit at the cost of
adding explicit checks to almost every single bit call, frequently writing
the same method twice and making iteration much much more difficult, does
not seem like a worthwhile tradeoff to me.

Also, rustc does not use Bitv anywhere, only through BitvSet, which does
not have this optimization.

For reference, here is some speed data from before and after this PR:

BEFORE:
test bitv::tests::bench_bitv_big        ... bench:     4 ns/iter (+/- 1)
test bitv::tests::bench_bitv_big_iter   ... bench:  4858 ns/iter (+/- 22)
test bitv::tests::bench_bitv_big_union  ... bench:   507 ns/iter (+/- 35)
test bitv::tests::bench_bitv_set_big    ... bench:     6 ns/iter (+/- 1)
test bitv::tests::bench_bitv_set_small  ... bench:     6 ns/iter (+/- 0)
test bitv::tests::bench_bitv_small      ... bench:     5 ns/iter (+/- 1)
test bitv::tests::bench_bitvset_iter    ... bench: 12930 ns/iter (+/- 662)
test bitv::tests::bench_btv_small_iter  ... bench:    39 ns/iter (+/- 1)
test bitv::tests::bench_uint_small      ... bench:     4 ns/iter (+/- 1)

AFTER:
test bitv::tests::bench_bitv_big        ... bench:     5 ns/iter (+/- 1)
test bitv::tests::bench_bitv_big_iter   ... bench:  5004 ns/iter (+/- 102)
test bitv::tests::bench_bitv_big_union  ... bench:   356 ns/iter (+/- 26)
test bitv::tests::bench_bitv_set_big    ... bench:     6 ns/iter (+/- 0)
test bitv::tests::bench_bitv_set_small  ... bench:     6 ns/iter (+/- 1)
test bitv::tests::bench_bitv_small      ... bench:     4 ns/iter (+/- 1)
test bitv::tests::bench_bitvset_iter    ... bench: 12918 ns/iter (+/- 621)
test bitv::tests::bench_btv_small_iter  ... bench:    50 ns/iter (+/- 5)
test bitv::tests::bench_uint_small      ... bench:     4 ns/iter (+/- 1)

Author: apoelstra