Since most patterns have less than 8 variables (usually, 1-4),
it's more efficient to use key-value slices instead of maps.
They also make memory reusing easier.

Even for 8 vars, slices are more efficient here.

```
name                           old time/op    new time/op    delta
Match/simpleLit-8                 217ns ± 3%     123ns ± 4%  -43.33%  (p=0.008 n=5+5)
Match/capture1-8                  652ns ± 7%     275ns ± 2%  -57.79%  (p=0.008 n=5+5)
Match/capture2-8                  950ns ± 0%     438ns ± 1%  -53.84%  (p=0.008 n=5+5)
Match/capture8-8                 3.89µs ± 5%    2.51µs ± 0%  -35.57%  (p=0.008 n=5+5)
Match/capture2same-8              947ns ± 2%     458ns ± 6%  -51.68%  (p=0.008 n=5+5)
Match/capture8same-8             3.42µs ± 2%    2.31µs ± 2%  -32.36%  (p=0.008 n=5+5)
Match/captureBacktrackLeft-8     5.87µs ± 1%    4.43µs ± 1%  -24.60%  (p=0.008 n=5+5)
Match/captureBacktrackRight-8    4.80µs ± 0%    2.43µs ± 3%  -49.42%  (p=0.008 n=5+5)

name                           old alloc/op   new alloc/op   delta
Match/simpleLit-8                 80.0B ± 0%     32.0B ± 0%  -60.00%  (p=0.008 n=5+5)
Match/capture1-8                   416B ± 0%       32B ± 0%  -92.31%  (p=0.008 n=5+5)
Match/capture2-8                   464B ± 0%       32B ± 0%  -93.10%  (p=0.008 n=5+5)
Match/capture8-8                   896B ± 0%      128B ± 0%  -85.71%  (p=0.008 n=5+5)
Match/capture2same-8               464B ± 0%       32B ± 0%  -93.10%  (p=0.008 n=5+5)
Match/capture8same-8               896B ± 0%      128B ± 0%  -85.71%  (p=0.008 n=5+5)
Match/captureBacktrackLeft-8     2.75kB ± 0%    0.85kB ± 0%  -69.19%  (p=0.008 n=5+5)
Match/captureBacktrackRight-8    2.54kB ± 0%    0.43kB ± 0%  -83.02%  (p=0.008 n=5+5)

name                           old allocs/op  new allocs/op  delta
Match/simpleLit-8                  2.00 ± 0%      1.00 ± 0%  -50.00%  (p=0.008 n=5+5)
Match/capture1-8                   4.00 ± 0%      1.00 ± 0%  -75.00%  (p=0.008 n=5+5)
Match/capture2-8                   5.00 ± 0%      1.00 ± 0%  -80.00%  (p=0.008 n=5+5)
Match/capture8-8                   15.0 ± 0%       4.0 ± 0%  -73.33%  (p=0.008 n=5+5)
Match/capture2same-8               5.00 ± 0%      1.00 ± 0%  -80.00%  (p=0.008 n=5+5)
Match/capture8same-8               15.0 ± 0%       4.0 ± 0%  -73.33%  (p=0.008 n=5+5)
Match/captureBacktrackLeft-8       31.0 ± 0%      21.0 ± 0%  -32.26%  (p=0.008 n=5+5)
Match/captureBacktrackRight-8      25.0 ± 0%      12.0 ± 0%  -52.00%  (p=0.008 n=5+5)
```