This speeds up the ascii case (and long stretches of ascii in otherwise
mixed UTF-8 data) when checking UTF-8 validity.

Benchmark results suggest that on purely ASCII input, we can improve
throughput (megabytes verified / second) by a factor of 13 to 14!
On xml and mostly english language input (en.wikipedia xml dump),
throughput increases by a factor 7.

On mostly non-ASCII input, performance increases slightly or is the
same.

The UTF-8 validation is rewritten to use indexed access; since all
access is preceded by a (mandatory for validation) length check, they
are statically elided by llvm and this formulation is in fact the best
for performance. A previous version had losses due to slice to iterator
conversions.

A large credit to Björn Steinbrink who improved this patch immensely,
writing this second version.

Benchmark results on x86-64 (Sandy Bridge) compiled with -C opt-level=3.

Old code is `regular`, this PR is called `fast`.

Datasets:

- `ascii` is just ascii (2.5 kB)
- `cyr` is cyrillic script with ascii spaces (5 kB)
- `dewik10` is 10MB of a de.wikipedia xml dump
- `enwik10` is 100MB of an en.wikipedia xml dump
- `jawik10` is 10MB of a ja.wikipedia xml dump

```
test from_utf8_ascii_fast        ... bench:         140 ns/iter (+/- 4) = 18221 MB/s
test from_utf8_ascii_regular     ... bench:       1,932 ns/iter (+/- 19) = 1320 MB/s
test from_utf8_cyr_fast          ... bench:      10,025 ns/iter (+/- 245) = 511 MB/s
test from_utf8_cyr_regular       ... bench:      12,250 ns/iter (+/- 437) = 418 MB/s
test from_utf8_dewik10_fast      ... bench:   6,017,909 ns/iter (+/- 105,755) = 1740 MB/s
test from_utf8_dewik10_regular   ... bench:  11,669,493 ns/iter (+/- 264,045) = 891 MB/s
test from_utf8_enwik8_fast       ... bench:  14,085,692 ns/iter (+/- 1,643,316) = 7000 MB/s
test from_utf8_enwik8_regular    ... bench:  93,657,410 ns/iter (+/- 5,353,353) = 1000 MB/s
test from_utf8_jawik10_fast      ... bench:  29,154,073 ns/iter (+/- 4,659,534) = 340 MB/s
test from_utf8_jawik10_regular   ... bench:  29,112,917 ns/iter (+/- 2,475,123) = 340 MB/s
```

Co-authored-by: Björn Steinbrink <bsteinbr@gmail.com>

We need to guard that `len` is large enough for the fast skip loop.