Exploiting the fact that getting the length of the slices is known, we
can use a counted loop instead of iterators, which means that we only
need a single counter, instead of having to increment and check one
pointer for each iterator.
Benchmarks comparing vectors with 100,000 elements:
Before:
```
running 8 tests
test eq1_u8 ... bench: 66,757 ns/iter (+/- 113)
test eq2_u16 ... bench: 111,267 ns/iter (+/- 149)
test eq3_u32 ... bench: 126,282 ns/iter (+/- 111)
test eq4_u64 ... bench: 126,418 ns/iter (+/- 155)
test ne1_u8 ... bench: 88,990 ns/iter (+/- 161)
test ne2_u16 ... bench: 89,126 ns/iter (+/- 265)
test ne3_u32 ... bench: 96,901 ns/iter (+/- 92)
test ne4_u64 ... bench: 96,750 ns/iter (+/- 137)
```
After:
```
running 8 tests
test eq1_u8 ... bench: 46,413 ns/iter (+/- 521)
test eq2_u16 ... bench: 46,500 ns/iter (+/- 74)
test eq3_u32 ... bench: 50,059 ns/iter (+/- 92)
test eq4_u64 ... bench: 54,001 ns/iter (+/- 92)
test ne1_u8 ... bench: 47,595 ns/iter (+/- 53)
test ne2_u16 ... bench: 47,521 ns/iter (+/- 59)
test ne3_u32 ... bench: 44,889 ns/iter (+/- 74)
test ne4_u64 ... bench: 47,775 ns/iter (+/- 68)
```
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