Improve PartialEq for slices
Improve PartialEq for slices

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)
```

vhbit/rust,nwin/rust,aidancully/rust,dwillmer/rust,vhbit/rust,zubron/rust,jroesch/rust,graydon/rust,vhbit/rust,vhbit/rust,dwillmer/rust,graydon/rust,jroesch/rust,l0kod/rust,l0kod/rust,l0kod/rust,l0kod/rust,jashank/rust,graydon/rust,zubron/rust,graydon/rust,nwin/rust,aidancully/rust,reem/rust,jroesch/rust,aidancully/rust,zubron/rust,dwillmer/rust,nwin/rust,jroesch/rust,zubron/rust,pshc/rust,reem/rust,dwillmer/rust,pshc/rust,l0kod/rust,dwillmer/rust,jashank/rust,pshc/rust,l0kod/rust,zubron/rust,jroesch/rust,zubron/rust,aidancully/rust,jashank/rust,GBGamer/rust,GBGamer/rust,nwin/rust,aidancully/rust,l0kod/rust,jashank/rust,vhbit/rust,reem/rust,pshc/rust,GBGamer/rust,graydon/rust,jashank/rust,jroesch/rust,graydon/rust,GBGamer/rust,vhbit/rust,dwillmer/rust,nwin/rust,dwillmer/rust,jashank/rust,nwin/rust,l0kod/rust,vhbit/rust,zubron/rust,jroesch/rust,pshc/rust,GBGamer/rust,GBGamer/rust,jroesch/rust,jashank/rust,reem/rust,nwin/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,dwillmer/rust,pshc/rust,pshc/rust,reem/rust,reem/rust,pshc/rust,zubron/rust,vhbit/rust,nwin/rust,jashank/rust