Optimize Aho-Corasick root searches with a skip loop

[josevalim]

We introduce a loop for finding the root of Aho-Corasick state,
which allows it to be better optimized by the compiler.
This shows a 1.67x improvement over the previous algorithm.

Additionally, the root finding loop may be replaced by a SIMD
loop when looking up to 16 bytes on supported platforms.
On supported platforms, the SIMD operation costs 16-bytes
of memory when compiling the pattern.

Benchmarks were run on arm64 macOS. Full-subject searches use a
4 MiB zero-filled subject, 100 searches per sample, and the median
of five samples. Values are searches per second. Counts through 16
use SIMD. The 32-roots measures the scalar skip-loop improvement
independently.

No match:

 roots    before    after    speedup
     1       261     4228     16.20x
     2       262     3414     13.03x
     3       261     2101      8.05x
     4       260     1805      6.94x
     8       258     1147      4.45x
     9       260     1052      4.05x
    16       259      666      2.57x
    32       258      430      1.67x

First match immediately after the 4 MiB prefix:

 roots    before    after    speedup
     1       261     4218     16.16x
     2       261     3354     12.85x
     3       261     2072      7.94x
     4       261     1780      6.82x
     8       260     1141      4.39x
     9       261     1039      3.98x
    16       260      663      2.55x
    32       261      430      1.65x

Pattern compilation uses 100,000 compilations per sample
and the median of seven samples. Values are compilations
per second:

 roots    before    after    change
     2    628559   647903     +3.1%
     3    580009   577474     -0.4%
     4    516172   529459     +2.6%
     8    393026   390299     -0.7%
     9    367666   364475     -0.9%
    16    257562   255865     -0.7%
    32    150395   149792     -0.4%

[github-actions]

CT Test Results

    4 files    225 suites   1h 56m 8s ⏱️
3 885 tests 3 756 ✅ 129 💤 0 ❌
4 970 runs  4 818 ✅ 152 💤 0 ❌

Results for commit bb82b27.

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