Using pointer arithmetic did not help.

Before (as submitted):
```
test str::bench::from_utf8_lossy_100_ascii                      ... bench:       261 ns/iter (+/- 32)
test str::bench::from_utf8_lossy_100_multibyte                  ... bench:       440 ns/iter (+/- 48)
test str::bench::from_utf8_lossy_invalid                        ... bench:        89 ns/iter (+/- 5)
```

After:
```
test str::bench::from_utf8_lossy_100_ascii                      ... bench:       272 ns/iter (+/- 32)
test str::bench::from_utf8_lossy_100_multibyte                  ... bench:       447 ns/iter (+/- 48)
test str::bench::from_utf8_lossy_invalid                        ... bench:        95 ns/iter (+/- 9)
```