deque: Fix grow condition in add_front

Without this, it will hit the assert in fn grow after 32 consecutive
add_front.
deque: Add tests and bench tests

Add a test that excercises deque growing.

Add bench tests for grow, new, add_back, add_front, to expose how slow
these functions are.
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)
deque: Implement FromIterator

So that deque can be used with IteratorUtil::collect()
deque: Remove rendundant field hi

The deque is determined by vec self.elts.len(), self.nelts, and self.lo,
and self.hi is calculated from these.

self.hi is just the raw index of element number `self.nelts`
deque: Add tests for mut_iter and mut_rev_iter
deque: Change iterators to use the same index calculation as Deque

The previous implementation of reverse iterators used modulus (%) of
negative indices, which did work but was fragile due to dependency on
the divisor.
deque: Move the shorter part when growing

The deque is split at the marker lo, or logical index 0. Move the
shortest part (split by lo) when growing. This way add_front is just as
fast as add_back, on average.
deque: Implement Deque::with_capacity. Lower initial capacity to 8.

We need a reasonably small initial capacity to make Deques faster
for the common case.
deque: Remove obsolete methods .each() and .eachi()
deque: Implement Clone and Eq for Deque
auto merge of #7562 : blake2-ppc/rust/deque, r=cmr

Fix an assertion in grow when using add_front.

Speeds up grow (and the functions that use it) by a lot. We retain the vector instead of creating it anew for each grow. 

The struct field hi is removed since it is redundant, and all iterators are updated to use a representation closer to the Deque itself, it should make it work even if deque sizes are not powers of two.

Deque::with_capacity is also implemented, but .reserve() is not yet done.