internal/gogrep: use external MatcherState

[quasilyte]

The reason we have to clone rule set every time we do Engine.Run
is gogrep.Pattern internal state. Every gogrep matcher has mutable
state that can't be combined with the thread-safe API of Engine.Run.

Cloning is usually not very expensive, we just copy a few dozens
of slices here and there, but it's noticeable for both small and
big rule file sizes.

If we move the mutable shared state out of the matchers and keep
it somewhere in the per-run or per-context scope, we should be
able to reuse it without races and there will be no need to
copy the matchers/patterns on their own as their internal structure
will become immutable (they receive a mutable state via param).

My experiments show ~15% speedup on the current stage.
I may move gogrep state even higher, to the per-context scope
so it's reused even more efficiently. But as a proof of concept
we can see that thread-safe version of Engine.Run is possible
without excessive copying.

With small rules file and small target file:

name         old time/op    new time/op    delta
EngineRun-8    41.4µs ± 3%    21.5µs ± 1%  -48.05%  (p=0.000 n=10+10)

name         old alloc/op   new alloc/op   delta
EngineRun-8    17.1kB ± 0%     3.4kB ± 0%  -80.14%  (p=0.000 n=10+10)

name         old allocs/op  new allocs/op  delta
EngineRun-8       129 ± 0%        47 ± 0%  -63.57%  (p=0.000 n=10+10)

With big rules file and small targe file:

name         old time/op    new time/op    delta
EngineRun-8     968µs ± 2%     481µs ± 4%  -50.37%  (p=0.000 n=9+9)

name         old alloc/op   new alloc/op   delta
EngineRun-8     429kB ± 0%      13kB ± 0%  -96.94%  (p=0.000 n=10+10)

name         old allocs/op  new allocs/op  delta
EngineRun-8     2.64k ± 0%     0.31k ± 0%  -88.33%  (p=0.000 n=10+10)

With small rules file and big target file size:

name         old time/op    new time/op    delta
EngineRun-8     127µs ± 5%     105µs ± 1%  -17.15%  (p=0.000 n=9+9)

name         old alloc/op   new alloc/op   delta
EngineRun-8    22.0kB ± 0%     8.3kB ± 0%  -62.27%  (p=0.000 n=10+10)

name         old allocs/op  new allocs/op  delta
EngineRun-8       267 ± 0%       185 ± 0%  -30.71%  (p=0.000 n=10+10)

With huge rules file and big target file size:

name         old time/op    new time/op    delta
EngineRun-8    4.31ms ± 2%    3.72ms ± 1%  -13.71%  (p=0.000 n=10+9)

name         old alloc/op   new alloc/op   delta
EngineRun-8     463kB ± 0%      46kB ± 0%  -89.97%  (p=0.000 n=10+10)

name         old allocs/op  new allocs/op  delta
EngineRun-8     3.53k ± 0%     1.20k ± 0%  -66.01%  (p=0.000 n=10+10)

Fixes #285