collections: Use slice parts in PartialEq for VecDeque
This improves == for VecDeque by using the slice representation.
This will also improve further if codegen for slice comparison improves.
Benchmark run of 1000 u64 elements, comparing for equality (all equal).
Cpu time to compare the vecdeques is reduced to less than 50% of what it
was before.
```
test test_eq_u64 ... bench: 1,885 ns/iter (+/- 163) = 4244 MB/s
test test_eq_new_u64 ... bench: 802 ns/iter (+/- 100) = 9975 MB/s
```
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