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
```
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