deque: Speed up deque growth by a lot

Fix some issues with the deque being very slow, keep the same vec around
instead of constructing a new. Move as few elements as possible, so the
self.lo point is not moved after grow.

   [o o o o o|o o o]
       hi...^ ^.... lo

   grows to

   [. . . . .|o o o o o o o o|. . .]
              ^.. lo        ^.. hi

If the deque is append-only, it will result in moving no elements on
grow. If the deque is prepend-only, all will be moved each time.

The bench tests added show big improvements:

Timed using `rust build -O --test extra.rs && ./extra --bench deque`

Old version:

test deque::tests::bench_add_back ... bench: 4976 ns/iter (+/- 9)
test deque::tests::bench_add_front ... bench: 4108 ns/iter (+/- 18)
test deque::tests::bench_grow ... bench: 416964 ns/iter (+/- 4197)
test deque::tests::bench_new ... bench: 408 ns/iter (+/- 12)

With this commit:

test deque::tests::bench_add_back ... bench: 12 ns/iter (+/- 0)
test deque::tests::bench_add_front ... bench: 16 ns/iter (+/- 0)
test deque::tests::bench_grow ... bench: 1515 ns/iter (+/- 30)
test deque::tests::bench_new ... bench: 419 ns/iter (+/- 3)

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