[libzstd] Speed up single segment zstd_fast by 5%

[libzstd] Speed up single segment zstd_fast by 5%

This PR is based on top of PR #1563.

The optimization is to process two input pointers per loop.
It is based on ideas from [igzip] level 1, and talking to @gbtucker.

| Platform                | Silesia     | Enwik8 |
|-------------------------|-------------|--------|
| OSX clang-10            | +5.3%       | +5.4%  |
| i9 5 GHz gcc-8          | +6.6%       | +6.6%  |
| i9 5 GHz clang-7        | +8.0%       | +8.0%  |
| Skylake 2.4 GHz gcc-4.8 | +6.3%       | +7.9%  |
| Skylake 2.4 GHz clang-7 | +6.2%       | +7.5%  |

Testing on all Silesia files on my Intel i9-9900k with gcc-8

| Silesia File | Ratio Change | Speed Change |
|--------------|--------------|--------------|
| silesia.tar  | +0.17%       | +6.6%        |
| dickens      | +0.25%       | +7.0%        |
| mozilla      | +0.02%       | +6.8%        |
| mr           | -0.30%       | +10.9%       |
| nci          | +1.28%       | +4.5%        |
| ooffice      | -0.35%       | +10.7%       |
| osdb         | +0.75%       | +9.8%        |
| reymont      | +0.65%       | +4.6%        |
| samba        | +0.70%       | +5.9%        |
| sao          | -0.01%       | +14.0%       |
| webster      | +0.30%       | +5.5%        |
| xml          | +0.92%       | +5.3%        |
| x-ray        | -0.00%       | +1.4%        |

Same tests on Calgary. For brevity, I've only included files
where compression ratio regressed or was much better.

| Calgary File | Ratio Change | Speed Change |
|--------------|--------------|--------------|
| calgary.tar  | +0.30%       | +7.1%        |
| geo          | -0.14%       | +25.0%       |
| obj1         | -0.46%       | +15.2%       |
| obj2         | -0.18%       | +6.0%        |
| pic          | +1.80%       | +9.3%        |
| trans        | -0.35%       | +5.5%        |

We gain 0.1% of compression ratio on Silesia.
We gain 0.3% of compression ratio on enwik8.
I also tested on the GitHub and hg-commands datasets without a dictionary,
and we gain a small amount of compression ratio on each, as well as speed.

I tested the negative compression levels on Silesia on my
Intel i9-9900k with gcc-8:

| Level | Ratio Change | Speed Change |
|-------|--------------|--------------|
| -1    | +0.13%       | +6.4%        |
| -2    | +4.6%        | -1.5%        |
| -3    | +7.5%        | -4.8%        |
| -4    | +8.5%        | -6.9%        |
| -5    | +9.1%        | -9.1%        |

Roughly, the negative levels now scale half as quickly. E.g. the new
level 16 is roughly equivalent to the old level 8, but a bit quicker
and smaller.  If you don't think this is the right trade off, we can
change it to multiply the step size by 2, instead of adding 1. I think
this makes sense, because it gives a bit slower ratio decay.

[igzip]: https://github.com/01org/isa-l/tree/master/igzip