collections: Hash VecDeque in its slice parts

Use .as_slices() for a more efficient code path in VecDeque's Hash impl.

This still hashes the elements in the same order.

Before/after timing of VecDeque hashing 1024 elements of u8 and
u64 shows that the vecdeque now can match the Vec
(test_hashing_vec_of_u64 is the Vec run).

before

test test_hashing_u64        ... bench:  14,031 ns/iter (+/- 236) = 583 MB/s
test test_hashing_u8         ... bench:   7,887 ns/iter (+/- 65) = 129 MB/s
test test_hashing_vec_of_u64 ... bench:   6,578 ns/iter (+/- 76) = 1245 MB/s

after

running 5 tests
test test_hashing_u64        ... bench:   6,495 ns/iter (+/- 52) = 1261 MB/s
test test_hashing_u8         ... bench:     851 ns/iter (+/- 16) = 1203 MB/s
test test_hashing_vec_of_u64 ... bench:   6,499 ns/iter (+/- 59) = 1260 MB/s

0 files changed, 0 insertions, 0 deletions